Bingo! How to post to Lemmy from Mastodon!
Put the title of the post in the first line (or at least before any other IDs/users/groups). Then tag the lemmy community as though they are an account.
Then tag other people at the end if you want them included.
@paulschoe @test @piratepost @fediversenews @alghost
So, just to demonstrate that this is working, Troy can view this post on lemmy here: https://lemmy.ml/comment/446959
Otherwise, to answer your question, you’ve anyway done it: @test
Another would be @asklemmy (see https://lemmy.ml/c/asklemmy)
Mastodons autocomplete should help you type these community names and addresses.