Create a Udemy course, with a use case that would clearly need and leverage your app. Offer that course for free on communities like HN, Reddit, etc. Works very well.
Noted. Thanks, although I can't easily figure out a way to connect a Udemy course to my non-existing service/app. I guess some services will work better than others with this method. However this is something I'd never thought of, and I think it's brilliant in a way.