The custom calendar is not available in Outlook calendar triggers/actions of Microsoft Flow currently, only built-in calendars are supported currently.
i.e., when you setup a Flow which updates the Body of an Event when it is created:
Trigger: When a new event is created (V2)
Action: Update event (V2)
the custom exchange resource calendar (such as the meeting rooms) is not supported and you cannot update the resource (Action Update event V2 failed)