-Make sure your developer console is enabled. To do this go to Options, Keyboard/Mouse, Allow Developer Console, set to Enabled.

-You must be the lobby host. Hit your tilda (`) key to open up the console.

Enter: sv_search_key "relentless1"

You can do this either in the lobby (though I have never gotten this to work for me) or at the main menu.

-Fill your lobby, start the game, and enjoy!