Hi Sebastian,
there are only 2 ways to authenticate a user:
- Entering the username and password (this gives you the user token).
- Using the user token (obtained in the step above).
To achieve what you want to do, you should manually create a token for the user (Site administration > Plugins > Web services > Manage tokens) and use that token to authenticate the user in a custom app, the official app doesn't support this. Please notice that this is really insecure since anyone with the token can access your site as that user.
Kind regards,
Dani