This is exactly what I would like to achieve with Zitadel. Zitadel also doesn't support custom scopes, so that might be the reason why consent prompt is not working.
Is there any example or suggestion how to show a consent screen for 3rd party apps?
Real world scenario: In our case we have external client that has its own pool of a users and provides mobile app. From xy mobile app client wants to access our resource server, but only if our user gives a consent to it - so in that app there would be a button connect with xy company. Then xy mobile app opens inapp browser where our (Zitadel) login screen is shown. After login screen I would like to show consent screen where the user will grant an access to the resources.