Shortcut to turn ON notifications for an app that are typically OFF

I want to be able to enable Notifications for a specific app with one press for an app that is typically off. The Focus feature doesn't work in this use case because when I try to create a custom focus, the app doesn't appear as an option when I go to Create a custom Focus > Apps > Choose Apps to Allow/Add Apps. I think the problem is that the custom focus setup assumes that you only want to manage apps that are typically using notifications.

UPDATE - as I typed the last paragraph I had an idea for a workaround. I enabled Notifications on the app in question, THEN successfully created my Focus, then disabled notifications on that app. The Custom Focus does appear to retain the settings I want. Will need to do some testing... Further input welcome. :-)