It's less of a feature and more of a convention. Just put "Show HN:" in front of the title if you're showing your work, or "Ask HN:" if you're asking a question without linking to an existing article. For "Show HN", it's customary to only submit each project once.

There is also "Tell HN:" but afaik it's mostly used by the mods, dang and sctb, to announce rule changes or new features.