• Choose “Other” for the service
• Set “http://proxy.listenbrainz.org” as the URL
• Use your MusicBrainz username as username
• Use your ListenBrainz user token as password (get it on https://listenbrainz.org/profile/)

Source: https://github.com/quodlibet/quodlibet/issues/2397#issuecomment-362596040