In your server.cfg file, you need to set:

sv_search_key "my key here"

In game, you need to make sure you have Console enabled:

Options -> Keyboard -> Enable Developer Console

After which you can start a game on your server via:

Console -> sv_search_key "my key here" -> Start a lobby -> Setup game & invite friends -> "START THE GAME"

If you wish to have the search key enabled, you need to make sure:

  • There is no Game Password / sv_password
  • sv_steamgroup_exclusive is set to 0