Can't add computer to iTunes Match (2021)

I have an old iTunes Match subscription but never got around to enabling it after the dawn of Apple Music.

Wanted to listen to some old playlists today but got stuck trying to enable iTunes Match on Apple Music on Big Sur. Followed these official directions but had the same issue as https://discussions.apple.com/thread/251409123 and https://discussions.apple.com/thread/250638734: the app freezes when clicking 'Add this computer'

After some trouble shooting I figured out the solution so wanted to share. Hope it's helpful! 😄