Threads like "Is there a tool that does X?" or "Alternatives to Y?" convert far better than ads: the person already wants a solution. Here is a method that works for a small SaaS or side project.
Use Reddit search (sorted by new, last week) and HN Algolia with patterns: "alternative to", "looking for a tool", "recommend a", "how do you handle", plus your category word. Ignore generic threads; keep those with a clear need.
Intent (are they asking for a tool?), freshness (under 48h), activity (a few replies, not 400) and fit (your product really solves it). Skip anything scoring low: a poor fit becomes spam.
Answer the question first, mention your product once and disclose that you built it. Read the subreddit rules before posting; many forbid links.
New threads appear every day and the first helpful replies get the attention. A 20-minute daily routine beats a one-off launch post.
Don't want to do the searching? ThreadRadar runs this scan every day and hands you the threads with a ready-to-post reply.