It can be frustrating if you're trying to have a conversation in a relaxed way, not so much proving your idea's worth (or yourself). That will invite thoughtful commenters but also haters and very critical people that aren't trying to contribute to your success... But that's life.

Just post as "Ask HN" and clarify you haven't finalized your ideas and that you're looking for constructive feedback.

Using prefix "Think HN" Looks like a good idea as a way to signal that but I'm afraid it would lose its appeal quickly.

IMHO, it's better to carefully craft your "Ask HN" submission in a way that invites the most useful feedback... And just ignore the rest.