How Reddit treats launch posts
Each subreddit sets its own rules on self-promotion: some allow product posts any day, some only in a weekly thread, some not at all. Many also filter posts from new or low-karma accounts. Read the sidebar and pinned posts before posting, and take part in discussions first.
Posts that do well tell a story and ask for something specific: what is confusing, what is missing, what people would pay for. Posts that read like an advertisement are downvoted or removed.
Writing for each community
Do not copy the same post to many subreddits. Rewrite the opening for each audience: a self-hosting community wants to know how it runs on their server, a marketing community what result it gets. Reddit titles can be up to 300 characters, but shorter titles that say what you made read better.
Questions
Which subreddits allow launch posts?
It changes by subreddit. Communities made for sharing projects, such as r/SideProject, welcome them; others limit self-promotion. Always read the rules first.
How long can a Reddit title be?
300 characters. Check yours with the Reddit title checker, but aim much shorter.
Should I include a link in the post?
Where the subreddit allows it, yes, tagged with UTM parameters so you can see what Reddit visitors did on your site.
Is anything I type sent anywhere?
No. The post is written in your browser and nothing is uploaded.