Have you added the “Content-type” header with a value of “application/json” when you call the users API in Mendix?
You can set this in the HttpHeaders tab of your “Call REST” action.
https://docs.mendix.com/refguide/call-rest-action/#http-headers
If this doesn’t work, try setting the log level of REST Consume to TRACE. This will log what your application is sending to the users API and you should be able to compare it to the request that works in Postman to see if there are any differences.
Good luck!