Don't think custom URL will work. If I put it in for Calendar it will take me to generic site and not have me within the new Google Calendar app. As I show no URL on my phone within the app. Is there a way to show it.
Custom URL means app specific URL protocol, e.g. fantastical2:// for Fantastical 2, or drafts4:// for Drafts 4. Not every app has registered one, or has one known in the public that works. Even if it works, i.e. can launch the app, it might not support other functionalities you might want but only launching the app. If you put a website URL (starting with http:// or https://) there, it will of course take you to Safari with the address you specified.