Windows doesn't send a global message when an application loses focus, so it's not possible to add as a Trigger event.

You could have a Window Focused Trigger that uses *.* as the application, and runs a Scripted Function that does this:

• Check if window that got focus was Spotify
• If not, check if Spotify is visible
• If so, minimize Spotify

That would be a bit resource intensive though, since it would have to run through that script every time a different window gets focus.