post https://api.themoviedb.org/3/authentication/token/validate_with_login
This method allows an application to validate a request token by entering a username and password.
Not all applications have access to a web view so this can be used as a substitute.
Please note, the preferred method of validating a request token is to have a user authenticate the request via the TMDB website. You can read about that method here.
If you decide to use this method please use HTTPS.
This would replace step 3 from the How do I generate a session id? guide.