What's a scenario file and what do you mean by it getting called?
If you mean "scene file" and "loading", then setting up a loading screen is a simple matter of just having a near-empty scene as the target chosen for JoinChannel. Inside that scene would be your loading screen UI, and a script that will immediately pause packet processing in its Awake() (via TNManager.isActive = false) and additively load your other scene while displaying its progress. Just don't forget to unpause the TNManager when you're done.
