Here is a Python script for doing what you want to do. Note there are two launch points one that is on the Allow Object Creation and the other on Initialize Value. I have named them PRLINE.ADD.VALIDATE and PRLINE.ADD.COPY respectively and those names do matter as they are used in the script to determine the correct behavior.

I have also included the Maximo Deployment Tools scriptConfig so you should be able to directly deploy the script using the VS Code extension https://marketplace.visualstudio.com/items?itemName=sharptree.maximo-script-deploy
and the launch points will be created for you correctly.

Let me know if you have any questions.

Jason

# As good practice define a main entry point to the script.# Focused on JavaScript but the principle remains.## https://www.sharptree.io/blog/2021/2021-11-03-js-functions/def main():    # There are two launch points, one to check if the record can be added    # the other to copy the value.    # In the Allow Object Creation launch point the global / implicit variable "mbo" does    # not exist and in the Initialize Value launch point the "mboset" variable does not exist.    # Define a common named variable called "check" and set it to the appropriate global variable.    check = None    if 'mboset' in globals():        check = mboset    elif 'mbo' in globals():        check = mbo    # Check that the mbo or mboset variable is available and that it is the expected PRLINE type.    if check is not None and check.isBasedOn("PRLINE"):        # Make sure we can get a reference to the owner and that it is a PR        if check.getOwner() is not None and check.getOwner().isBasedOn("PR"):            # If we are in the validate launch point then if the owner (PR) vendor is null then throw an error.            if launchPoint == "PRLINE.ADD.VALIDATE":                if check.getOwner().isNull("VENDOR"):                    service.error("msggroup","msgkey")            elif launchPoint == "PRLINE.ADD.COPY":                # if in the initialize if the Mbo is to be added then copy the vendor from the parent PR                if mbo.toBeAdded():                    check.setValue("CUSTOM_VENDOR", check.getOwner().getString("VENDOR"))# Call the main functionmain()# Configuration for the script and launch points to be deployed using the VS Code Maximo development tool extension# https://marketplace.visualstudio.com/items?itemName=sharptree.maximo-script-deployscriptConfig="""{    "autoscript": "PRLINE.ADD",    "description": "Automation script to validate the PR has a vendor and copy it if it does",    "version": "",    "active": true,    "logLevel": "ERROR",    "scriptLaunchPoints": [        {            "launchPointName": "PRLINE.ADD.VALIDATE",            "launchPointType": "OBJECT",            "active": true,            "description": "Automation script to validate the PR has a vendor",            "objectName": "PRLINE",            "allowObjectCreation": true        },        {            "launchPointName": "PRLINE.ADD.COPY",            "launchPointType": "OBJECT",            "active": true,            "description": "Automation script to copy the PR vendor to the line",            "objectName": "PRLINE",            "initializeValue": true        }            ]}"""

Here is a Python script for doing what you want to do. Note there are two launch points one that is on the Allow Object Creation and the other on Initialize Value. I have named them PRLINE.ADD.VALIDATE and PRLINE.ADD.COPY respectively and those names do matter as they are used in the script to determine the correct behavior.

I have also included the Maximo Deployment Tools scriptConfig so you should be able to directly deploy the script using the VS Code extension https://marketplace.visualstudio.com/items?itemName=sharptree.maximo-script-deploy
and the launch points will be created for you correctly.

Let me know if you have any questions.

Jason

# As good practice define a main entry point to the script.# Focused on JavaScript but the principle remains.## https://www.sharptree.io/blog/2021/2021-11-03-js-functions/def main():    # There are two launch points, one to check if the record can be added    # the other to copy the value.    # In the Allow Object Creation launch point the global / implicit variable "mbo" does    # not exist and in the Initialize Value launch point the "mboset" variable does not exist.    # Define a common named variable called "check" and set it to the appropriate global variable.    check = None    if 'mboset' in globals():        check = mboset    elif 'mbo' in globals():        check = mbo    # Check that the mbo or mboset variable is available and that it is the expected PRLINE type.    if check is not None and check.isBasedOn("PRLINE"):        # Make sure we can get a reference to the owner and that it is a PR        if check.getOwner() is not None and check.getOwner().isBasedOn("PR"):            # If we are in the validate launch point then if the owner (PR) vendor is null then throw an error.            if launchPoint == "PRLINE.ADD.VALIDATE":                if check.getOwner().isNull("VENDOR"):                    service.error("msggroup","msgkey")            elif launchPoint == "PRLINE.ADD.COPY":                # if in the initialize if the Mbo is to be added then copy the vendor from the parent PR                if mbo.toBeAdded():                    check.setValue("CUSTOM_VENDOR", check.getOwner().getString("VENDOR"))# Call the main functionmain()# Configuration for the script and launch points to be deployed using the VS Code Maximo development tool extension# https://marketplace.visualstudio.com/items?itemName=sharptree.maximo-script-deployscriptConfig="""{    "autoscript": "PRLINE.ADD",    "description": "Automation script to validate the PR has a vendor and copy it if it does",    "version": "",    "active": true,    "logLevel": "ERROR",    "scriptLaunchPoints": [        {            "launchPointName": "PRLINE.ADD.VALIDATE",            "launchPointType": "OBJECT",            "active": true,            "description": "Automation script to validate the PR has a vendor",            "objectName": "PRLINE",            "allowObjectCreation": true        },        {            "launchPointName": "PRLINE.ADD.COPY",            "launchPointType": "OBJECT",            "active": true,            "description": "Automation script to copy the PR vendor to the line",            "objectName": "PRLINE",            "initializeValue": true        }            ]}"""

Here is a Python script for doing what you want to do. Note there are two launch points one that is on the Allow Object Creation and the other on Initialize Value. I have named them PRLINE.ADD.VALIDATE and PRLINE.ADD.COPY respectively and those names do matter as they are used in the script to determine the correct behavior.

I have also included the Maximo Deployment Tools scriptConfig so you should be able to directly deploy the script using the VS Code extension https://marketplace.visualstudio.com/items?itemName=sharptree.maximo-script-deploy
and the launch points will be created for you correctly.

Let me know if you have any questions.

Jason

# As good practice define a main entry point to the script.
# Focused on JavaScript but the principle remains.
#
# https://www.sharptree.io/blog/2021/2021-11-03-js-functions/

def main():
    # There are two launch points, one to check if the record can be added
    # the other to copy the value.
    # In the Allow Object Creation launch point the global / implicit variable "mbo" does
    # not exist and in the Initialize Value launch point the "mboset" variable does not exist.
    # Define a common named variable called "check" and set it to the appropriate global variable.
    check = None

    if 'mboset' in globals():
        check = mboset
    elif 'mbo' in globals():
        check = mbo

    # Check that the mbo or mboset variable is available and that it is the expected PRLINE type.
    if check is not None and check.isBasedOn("PRLINE"):
        # Make sure we can get a reference to the owner and that it is a PR
        if check.getOwner() is not None and check.getOwner().isBasedOn("PR"):
            # If we are in the validate launch point then if the owner (PR) vendor is null then throw an error.
            if launchPoint == "PRLINE.ADD.VALIDATE":
                if check.getOwner().isNull("VENDOR"):
                    service.error("msggroup","msgkey")
            elif launchPoint == "PRLINE.ADD.COPY":
                # if in the initialize if the Mbo is to be added then copy the vendor from the parent PR
                if mbo.toBeAdded():
                    check.setValue("CUSTOM_VENDOR", check.getOwner().getString("VENDOR"))

# Call the main function
main()

# Configuration for the script and launch points to be deployed using the VS Code Maximo development tool extension
# https://marketplace.visualstudio.com/items?itemName=sharptree.maximo-script-deploy
scriptConfig="""{
    "autoscript": "PRLINE.ADD",
    "description": "Automation script to validate the PR has a vendor and copy it if it does",
    "version": "",
    "active": true,
    "logLevel": "ERROR",
    "scriptLaunchPoints": [
        {
            "launchPointName": "PRLINE.ADD.VALIDATE",
            "launchPointType": "OBJECT",
            "active": true,
            "description": "Automation script to validate the PR has a vendor",
            "objectName": "PRLINE",
            "allowObjectCreation": true
        },
        {
            "launchPointName": "PRLINE.ADD.COPY",
            "launchPointType": "OBJECT",
            "active": true,
            "description": "Automation script to copy the PR vendor to the line",
            "objectName": "PRLINE",
            "initializeValue": true
        }        
    ]
}"""

What version of Maximo are you on? I don't remember what version this was added in but if you're on an older 7.6.0.X version you may not have this event. Using .NEW is advantageous though because it fires once only when a record is being added and is done immediately. Jason's example will work on more versions but depends on initialize of the object which happens often. For example, when you copy the PRLINE to a POLINE we need to initialize the PRLINE object. He has the proper checks in place to avoid doing anything in that scenario but the script still will fire for each line.

The first check "if owner.getString("VENDOR") is None:" is something to be careful of with automation scripts in general. Sometimes getString() returns an empty string which is not the same as None. What I tell customers to do on python/jython scripts is keep it simple. 

if not owner.getString("VENDOR"):

That will handle empty strings as well. Or use our MBO method like Jason did:

if owner.isNull("VENDOR"):

That should fix the first issue. The second piece should work so if you're still having issues, I'd try to set a different field (like remark) to see if it's some logic on your custom field that's causing issues.

What version of Maximo are you on? I don't remember what version this was added in but if you're on an older 7.6.0.X version you may not have this event. Using .NEW is advantageous though because it fires once only when a record is being added and is done immediately. Jason's example will work on more versions but depends on initialize of the object which happens often. For example, when you copy the PRLINE to a POLINE we need to initialize the PRLINE object. He has the proper checks in place to avoid doing anything in that scenario but the script still will fire for each line.

The first check "if owner.getString("VENDOR") is None:" is something to be careful of with automation scripts in general. Sometimes getString() returns an empty string which is not the same as None. What I tell customers to do on python/jython scripts is keep it simple. 

if not owner.getString("VENDOR"):

That will handle empty strings as well. Or use our MBO method like Jason did:

if owner.isNull("VENDOR"):

That should fix the first issue. The second piece should work so if you're still having issues, I'd try to set a different field (like remark) to see if it's some logic on your custom field that's causing issues.

What version of Maximo are you on? I don't remember what version this was added in but if you're on an older 7.6.0.X version you may not have this event. Using .NEW is advantageous though because it fires once only when a record is being added and is done immediately. Jason's example will work on more versions but depends on initialize of the object which happens often. For example, when you copy the PRLINE to a POLINE we need to initialize the PRLINE object. He has the proper checks in place to avoid doing anything in that scenario but the script still will fire for each line.

The first check "if owner.getString("VENDOR") is None:" is something to be careful of with automation scripts in general. Sometimes getString() returns an empty string which is not the same as None. What I tell customers to do on python/jython scripts is keep it simple. 

if not owner.getString("VENDOR"):

That will handle empty strings as well. Or use our MBO method like Jason did:

if owner.isNull("VENDOR"):

That should fix the first issue. The second piece should work so if you're still having issues, I'd try to set a different field (like remark) to see if it's some logic on your custom field that's causing issues.