It's easier said than done, I suppose, but my first ~500 users for https://radius.to/ were from HN after hitting the front page. A year later, I still get a decent amount of traffic from that post.
I'd imagine you'd get pretty good results on both ProductHunt and Reddit as well if you can get enough upvotes, though the latter is pretty difficult to promote your products on these days unless it's legitimately relevant to the community you post in.