@francis-ducharme
To confirm, you are:
Sending the user to a page such as:
https://local.fusionauth.io/oauth2/authorize?client_id=85a03867-dccf-4882-adde-1a79aeec50df&response_type=code&redirect_uri=https%3A%2F%2Fthird.com
The user will click login with Google or be redirected automatically to Google (if using an idp_hint, for instance)
In this case, FusionAuth will redirect to https://third.com (example only) but could just as easily redirect to https://fourth.com depending on step one. In either case, all possible redirect URLs for your application need to be previously defined on the OAuth configuration for that application.
Also, we do have a few github issues allowing a wildcard to be defined for a redirect URL.
https://github.com/FusionAuth/fusionauth-issues/issues/437
With more context, I might be able to provide additional feedback. Depending on context, deeplinking might also be something worth exploring
https://www.youtube.com/watch?v=-vx5rdy-mvY
Thanks,
Josh