Marketing would be the answer. Depending on how much money and how well you're able to market will determine your ability to generate a relevant audience. Putting aside the rare cases in which a game goes viral, you need to generate content on a regular basis via social media and on-top of that look into building a good marketing campaign which is going to cost. Regardless of the above you should at the bare minimum have a social media presence and a Discord.
You should also ask yourself how your game enforces the need for community engagement, as in why would anyone want to build a community around your game? Does the game offer online competitive play? Do you have the option for custom content to be done by your community? Is the game still in development or you're improving the game while seeking community feedback?