The documentation for the MAS core APIs is here: Maximo Application Suite Admin APIs 9.0.x - IBM API Hub - IBM Developer
I linked to the add new user page but there are pages for all the public APIs here. Just be aware that when updating a user, it's very different than Manage APIs. In Manage you provide the identifier (such as USERID) and only the fields you want to change. In MAS you need to provide the full user payload, or it will assume you want to nullify the value. For example, if you don't provide the givenName (first name), it will be null.
Also be aware that these APIs change based on major version. I provided you the 9.0 documentation since you mentioned MAS core. 9.1 Manage takes over most of the user management.
------------------------------
Steven Shull
Naviam
------------------------------