FarmSim works a bit different compared to our other games, as ClanForge does not have any way to start the game server itself. Rather, it has to be started from inside FarmSim's own web admin panel. You can get to this from the server dashboard in ClanForge - just click the blue 'Webadmin' button in the server info panel.
You will need to log in with the username 'admin' and the password you have set in ClanForge. To find it, just log into ClanForge (if you aren't already) and click 'Profiles' -> 'Edit Active' in the sidebar. You will then see the password in the 'Admin Password' field.
Once you're logged in, click 'Start' - the button is at the bottom of the Configuration section of the FarmSim web admin home page - this will start the game server itself and you should then be able to find it from in-game.