LUNADIO is nice community for it. I recommend you to discuss your product on master mind one or 2 weeks before the launch.
https://www.indiehackers.com/ and twitter are also nice to use.
Depending on the platforms you usually use to engage with other entrepreneurs, founders, etc, and where you engage with your own community of potential users. I would say social platform like LinkedIn, Twitter, Facebook groups, Reddit, Indie Hackers. Maybe Hacker News?
Some of my favourite slack communities you can share PH launch on:
- This week in startups by Jason Calacanis (37k members) -
- Acquired (7k members)
- The Product Folks (7k members)
Lenny's Newsletter community is amazing, I believe you need to sign up for his newsletter to get access!
Sharing it on Twitter, HackerNews, HackerNoon and other sites is also great!