You have to slowly engage with the community and use it or a regular but even then you are not supposed to self promote. What you can do is ask for peoples opinion of your app.

Reddit is banning accounts so frequently. I would suggest avoid posting anything for the first week of account creation, and even when you post, do not add a link. unfortunately the only way for startups to promote themselves now is to do shadow marketing.

Some of us folks here did startups before there was Reddit (or HN). Launching for free wasn't a thing. So things turn around.