I would use the /maximo/api (the /maximo/oslc API without security constraints) instead of the legacy /maxrest/rest API. But if you want to use API keys with the legacy maxrest you would have to comment out the security constraints in the web.xml for the applications\maximo\maxrestweb\webmodule\WEB-INF. We didn't always support API keys for the MAXREST but I think we had added by 7.6.1.2. When we first added API keys it was only for the new API (/maximo/oslc) and then we started adding it to the other web services.
The reason you can't authenticate with an API key when you have the security constraints is the security constraints tell WebSphere to handle authentication but the API keys are specific to Maximo. By removing the security constraints, WebSphere won't try to authenticate the request and allows the request to make it to Maximo so that it can validate the API key.
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Steven Shull
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