How to get your #OpenStreetMap Account show up verified in your Mastadon profile:
(1) Open openstreetmap.org/profile/edit
(2) Add <a href="en.osm.town/@YOUR_NAME" rel="me">Mastadon</a>
(3) Open en.osm.town/settings/profile
(4) Remove Link and(!) Label of your existing OSM Profile link (if present), then save
(5) Re-add Label and Link
(6) Visit your Mastdon profile, the OSM link is now green.