I am guessing you have a good idea of who your likely customers could be - maybe you have an idea of who your marquee customers could be, and a list of smaller companies that could be your early adopters.

The best way is to reach out to these companies and talk to them and sign them up as beta customers for when you launch.

Of course, you could also post on forums like Hacker News or forums that are relevant to your customers to attract their attention. However, I feel a real conversation is the best way to get a feel for who will sign up as an active beta user.

Reach out via people in your network, or request for a short call to get feedback from your potential customers and try to turn them into beta users.