FoF Passport enables OAuth 2 flow from Flarum to the OAuth 2 server configured. In no way does it pre-authenticate users in Flarum based on a random third party application; that would be a severe security issue.
What you can do is have your (Angular) application (or rather its backend) pre authenticate the user, retrieve a unique token, then redirect the user to Flarum to a custom endpoint that reads and confirms the token, after which the user can be authenticated on Flarum.
OAuth2 will always redirect users to the OAuth2 server when trying to log in, based on whether or not you use implicit access the user is then redirected back to that application.
