@bloodaxe right, but you can still create an account on a friendica instance to create that forum, which you can follow from Mastodon (or any other #fediverse service that supports ActivityPub or any of the other protocols that Friendica supports) similarly with Guppe Groups, however my understanding is that you don't even need to create an acoount, and just mentioning one like @VideoGame@gup.pe will post to the group, or create it if doesn't exist. On Mastodon both options appear as if the group "user" re-toots every post to the group. Just an option in addition to hashtags so that others who might be interested in such could find it if looking there for particular topics rather than stumbling upon a specific hashtag.