It doesn’t really work very well as that. It’s buggy and it’s difficult to really understand. PieFed/Lemmy/Mbin works much better for the purpose intended by OP.

This does mean you’ll need to have separate accounts for Mastodon and Forumverse, though. At least for now. I tried using MBin to combine the two, but in practical terms the user experience was lacking, because Mbin is not quick enough in implementing new features that Mastodon gets. Or ones that PieFed and Lemmy get. Especially the images inside Forumverse posts showing as mere links bugged me like hell! And quote toots on Mastodon not working either.