Read the rules before you post
Every subreddit sets its own rules on self-promotion, links and post flair, usually in the sidebar or a pinned post. Some ban links entirely, some allow them only on certain days, some allow them if you disclose. Note them before your first comment.
Reddit's site-wide Content Policy also applies: no spam, no manipulating votes, no ban evasion.
Answer first, for weeks
Your first comments in a community should be useful on their own, with no link. Answer the question fully, share what you tried, and follow up when people reply. That history is what makes a later mention credible.
When you mention your product
Only when it fits the question. Say plainly that you built it, explain how it solves their exact problem, and mention trade-offs. One link at most. If the rules forbid links, describe it and let people ask.
What gets founders banned
Posting the same link across many subreddits, using a second account to praise your own product, asking friends to upvote, deleting and reposting, and arguing with moderators. None of it is worth the risk to your domain.
Measure what replies lead to
Give each reply that includes a link its own tracking link so you can see visits per comment, and label anything you can't match as unattributed. GTMorganic drafts Reddit replies from threads you paste (no Reddit API) and keeps a receipt for each step.