This tutorial gives you a guide about using command in order to activate cheats on DOOM 3: BFG Edition game:
To use ~ for command to use cheats:
Right click on game icon..open file location..
Open the "base" folder.
Press the Windows key + R to bring up the Windows run dialog.
Type "notepad" and press Enter to open Notepad.
In the open folder, drag the "default.cfg" file into Notepad.
Add the following line to the bottom of the text:
seta com_allowconsole 1
Note: This will disable achievements.
Click File -> Save and close Notepad.
Cheats (can be activated after bringing up the console)
give all, give ammo, god, give health etc.