The software you write, in C++, is the server. It uses something like cpp-httplib to open up a socket, listen on a port, and dispatch incoming requests.
The very front page of the github repository has an example of how to start it up:
You change the various Post and Get request handlers to do whatever your game needs.
Something like, POST to /game to create a new game instance, returning some game ID. POST to /game/GAMEID/move/PLAYERID to add a move in the game. Do a GET to /game/GAMEID to get the current state of the game.
