How to populate the server of a multiplayer game in itch.io
Host it on a platform that has inbuilt multiplayer support and some sort of friends system, so players can play with their real life and their online friends.
Itch is not such a platform.
If your game has anonymous multiplayer, you simply have the same problem as anyone else. Getting an audience. Good luck with that. You and several thousand other developers are trying to do this. It is just like hechelion explained. If it were easy, everyone would be doing it, and everyone would be equal again.