You usually let SDK, rather than the client application, choose a room server, and SDK chooses a room server at random.
In Strix Unity SDK, you don’t specify a room server when creating a room. The CreateRoom method internally chooses a room server at random and connects to it. To join an existing room, you first search for a room, and a search result contains the room server’s address information. You can just pass the address to the JoinRoom method, and it connects to that room server automatically.