Unclassified microblogs (e.g. from Mastodon users) usually end up in random, but I'm not sure how to post them intentionally since I don't use the microblog feature much. Hopefully someone else can chime in with an answer for this.
I can answer this, go to your own profile, choose add new post (not thread) and then from the "select a magazine" dropdown choose random. Then just use your @ tag like normal if you want to @ someone in the fediverse.