Whats your primary channel or hack to get waitlist users?

In the past, I used Facebook ads to a simple landing page with email signup. Mostly optimized for cheapest leads. Next time I do it, I will create tags and segments based on ad targeting so I can get better analytics later on what worked/didn't.

This depends a lot on the users you are targeting. If you have some which are organised in communities, Reddit or Discord worked quite well for us.