You learn where your customers hang out, and then you hang out there and authentically become part of the social group. You spend weeks/months/years learning what they like, their workflows, and their problems. Or, better, you've already spent years working with your customers so you have some experience in these matters. Then you spend more time setting up face-to-face discussions, at conferences, online, or wherever they hang out, *not* trying to shill, but honestly and authentically trying to learn what they need.
It may take dozens or hundreds of attempts, and then you find a small group, maybe a half dozen or so, that are early adopters, willing to live with your experiments and provide feedback. Work with them to hone the value proposition, and learn how to communicate it effectively. Tweak or pivot the product to fit their needs, often for many more months.
There is no simple solution that involves making a few social media posts, or paying for advertisements, or spamming people with email. Everything that actually works takes lots of personal time and energy.
