Gotcha, thanks for clarifying!
The most straightforward way to get an access token for a user is to use the resource owner password flow against the /oauth/token endpoint of the Authentication API:
https://auth0.com/docs/api/authentication#resource-owner-password
You’ll need to make sure whatever client_id you are setting has the password grant enabled in the application settings in your dashboard (settings → advanced settings → grant types).
You can add custom claims to the token with an Action which will execute when the user logs in:
Hope this helps!