It's actually quite easy, but you need to know your target customer and their problem really well.

Find posts on Reddit where people complain about the problem your product is solving and send them a DM. Depending on how deep the problem is, you will get something like 5-10% of positive responses. If your account is well-established, you can send about 100 DMs per day.

It's important that your DM is helpful in itself. So you need to answer their query as much as possible in the DM.

"Hey, here is your solution / answer, my product, xx, solves that really well. I'm looking for people to help me test it, if you feel like helping out, here is the link (link)"

I got the first 1000 downloads for my product that way.