I wrote about this fairly extensively:
TL;DR: Email sign ups before launch via a boosted IG post.
Non TL;DR: https://www.swiftjectivec.com/the-first-100-subscribers/
What we are currently doing: 1) Cold outreach to power users - to convert them into affiliates. 2) Cold outreach to individuals who have target ICP communities. 3) SEO for more long term (not for the first 500)
TL;DR: Email sign ups before launch via a boosted IG post.
Non TL;DR: https://www.swiftjectivec.com/the-first-100-subscribers/