If you want to share to the OSM.town Mapstodon server (for people logged into there), you can open a pop-up with the following URL. Logged in users (to mastodon) will see a post prefilled with the text. I think this is similar to how the Nazi-Bar Site Twitter sharing is done.
https://en.osm.town/share?text=TEXT&url=URL
e.g. (using the “share this post on X” for this post would mean this url: https://en.osm.town/share?url=https%3A%2F%2Fcommunity.openstreetmap.org%2Ft%2Fsrsly-sharing-selections-to%2F122144&text=SRSLY+sharing+selections+to) and shows me a window like:
