add placeholders at internal host

[smoebody]

Is your feature request related to a problem? Please describe.
My app is userless, so I use authentik-proxy for authenticating my user.
To enable multiple users i like to deploy multiple instances for the app - one for each user, e.g.

http://app-foo/
http://app-bar/
http://app-john/

But right now i can only forward an authenticated user to the same internal host.

Describe the solution you'd like
If i could make the internal host somewhat dynamic by adding a placeholder for the username i could proxy every user to its own application instance. e.g.:

http://app-{preferred_username}/

Describe alternatives you've considered
none

Additional context
I am using authentik-proxy to authenticate an authless app. Now while this works for authentication I am struggling with identification.

Thanks in advance

cheers