Whoever is running this server gave you a secret out of band (a chat message, not a public page). It is kept in this browser only, never shown back, and sent with every lobby request.
You become the host — you choose the mode and, once everyone's seated, start the game. 2 to 4 players.
Every chair has to be taken before the game can start — by a person or by a bot. Pick the size of the table now; it cannot change once the game is created.
Ask whoever created the game for its code.
They enter it under Join a game, along with the same join secret you used.
The host may fill an empty seat with a bot, and starts the game once every seat is either a player or a bot. These chairs are not the running order — who sits where along the Division is decided by a D12 roll when the game starts (§4.4), and the map shows the result.