How do you apply cheat codes to doom3 bfg edition. I tried using alt+ctrl+~ but no go. Can cheat codes still be applied. If so how, because the alt+ctrl+~ didn't work

You must enable the console with the launch argument +set com_allowconsole 1. Once you open the game press ~ and the console will show up, from there you can input any cheat code you like.

P.S.: Enabling the console disables the ability to unlock achievements due to cheating