Start the Epic launcher. On the left side near the bottom is Settings. Click on it, then scroll to the bottom to find the name of the game. Click on the name of the game to get some options.
Click on Additional Command Line Arguments to select it and get a box where you can enter the argument(s). Type cheat in the box. You don't need anything but those 5 letters.
When finished, go up to the top of the screen and click on SETTINGS to get back to the main screen.