Hi,
This seems to be a gap in the documentation and we apologize for this and will ensure it is added.
You can see what is needed to submit your app on this page https://www.ibm.com/support/pages/publishing-apps.
I will guide now on the basis that you have access to https://apps.xforce.ibmcloud.com/ with an account. Once you login, on the top left side of your screen you will see a hamburger menu 
click this and the menu will expand.

Next, click on Manage and then Submission Portal. You will see a page like the following:

from here, click on New Community App to start the process.
Please add to this thread if you need further assistance.
Thanks,
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Ben Clarke
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