From my experience, you should first understand your audience, who they are and then find places where they hang out. Find groups on Reddit, LinkedIn and Facebook and post about your product.

Another thing you could try is services like Betalist. For my last project it worked pretty well: i spent 120 EUR on paid promotion and got around 250 signups. I can't say that they were all relevant to my product, but around 30% signed up for the app and used it at least ones.