How to add userAdmin user in Mongodb Atlas
You cannot grant this role. The only way to create new MongoDB users in Atlas is either via the UI or the API. See Add MongoDB Users for details and note that this page states:


  Atlas rolls back any user modifications not made through the UI or API. You must use the Atlas UI or API to add, modify, or delete MongoDB users on Atlas clusters.


See Create a Database User for details on how to create MongoDB users via the API.