Subreddit rules vary but the universal pattern: post 5-10 useful, non-promotional comments BEFORE you ever post your link. Build karma in the sub first. When you do post, the format matters more than the content: "I built X to solve Y" works, "Check out my new app" doesn't. r/SideProject and r/IndieDev are more tolerant; r/programming is brutal.
Prompt to paste
I want to share [product] on Reddit without getting removed as self-promotion. Suggest 2-3 subreddits that fit, the karma-building comments I should make first, and an 'I built X to solve Y' style post (not 'check out my app') tailored to each sub's culture.