How to permanently disable Apple Music pop-up on iMac

I think this might be what you're looking for? At least this is the popup that keeps appearing every time I switch to another app and return to Music. Incredibly annoying. It's done this through numerous OS updates. I'm guessing there is some bit that isn't being reset when I click Start Listening, so it returns all the time. Switching to player mode by clicking the album art in the info window at the top stops this behavior as long as the player is open. Going back to the full app window restarts this popup appearing. Any idea how I can switch this off?

Unlike iPad/iPhone app settings which have many/most in the Settings app, in macOS most/all of an app's settings are within the app. In the app's window click on the app's name, drag down to Settings. Sometimes it is called by its old name, Preferences.

I am unaware of anything related to popups. What does the one you're getting say?

Again, what does it say?

Screen shot of the pop up may help. In some cases, simply clicking "Start Listening" will make the pop up go away and not come back.