To force your lobby to join your server by default you have two options
1) whilst in your lobby in the game console "tild" key type mm_dedicated_force_servers "51.89.176.80:27015" (your server ip:port) enter
2) then press start and your lobby will auto create on your server
or
3) You need to create an autoexec.cfg in C:\Program Files (x86)\Steam\SteamApps\common\left 4 dead 2\left4dead2\cfg and in that autoexec.cfg you will put the following
mm_dedicated_force_servers "xx.xx.xx.xx:xxx" replacing xx with server ip:port
this will auto force your lobby to your server every time you start left4dead2 lobbies
