Is this BIM 360 API question? This forum is for BIM 360 specific questions.
If you are using 3-legged authentication method, limiting to a specific user is as designed.
Some API call allows you to use 2-legged authentication although not all. For the part 2-legged is available, you may use it.
If you are trying to automate login process for your automated test, what you can do is to login as a user for the first time, then use refresh token for subsequence calls. For information about refresh token, you may refer to these posts:
https://forge.autodesk.com/blog/more-about-refresh-token
https://forge.autodesk.com/blog/about-refresh-token
Mikako Harada
Developer Technical Services