How to extract the content of a specific ID3 "User Defined Text" tag?

[Phil Harvey]

Note that I think I will change the family 1 group from UserDefined back to ID3v2_4 (or whatever) for ExifTool 12.65.  So you should drop the group name from your command, and just use:

exiftool -s3 -MusicBrainzAlbumArtistId music.mp3

I've also dropped the -a because there shouldn't be more than one of these.

- Phil