Indie Hackers is a forum where creators spam their newly built service. The spamming is often disguised by asking a question.

I think the good way to get your first users is to go to the very specific niche communities where they usually talk, hang out, and somehow incentivize them to try your thing out. Then ask them what they think, why they stopped using it, what they liked about it, etc.

Guerilla marketing methods can be very effective as well, but requires some creativity and sometimes luck.