On which subreddit can I promote an adult website?

Kinda feel this a question for Reddit.

Most subreddits do not allow ads. However, you might be able to get your question answered at r/RedditAds. You can’t advertise there, but it is a place where you can learn about Reddit advertising.

How specific is it? If it’s a niche interest or specific community, you want to join those communities on Reddit and take part in them anyway – you will quickly get a sense of what the norms are for how to share your site. If it’s a broader / general porn site you could share clips on the big subreddits such as /nsfwhardcore and /porn – those aren’t about community as much but you might get some traffic from organic shares there