How can I get principle for a specific user in Spring Boot?

I am pretty new to backend dev. So, I am trying to generate a token for a specific userId. But the URL doesn't have any query parameters or headers for userId. When I debug the token API, I see that the principal object contains a userId. But I want to change that userId. And I can't seem to figure out how. I read this thread to understand what's a principle but it says you can't pick a specific user. Any help is appreciated.