How do you activate Cheat Mode?

Cheat Mode can be unlocked during the match Prologue. There is a secret screen to the left of where you begin the region, including Madeline’s automobile and a few street lamps. On this display, you have to enter a special button combination.

The cheat code

The cheat code consists of the following actions:
Left, Right, Journal, Climb, Up, Up, Down, Left, Climb, Confirm

For a keyboard with default controls, this would translate to:
Left, Right, Tab, Z, Up, Up, Down, Left, Z, C

On an Xbox controller with default controls, this would translate to:
Left, Right, LT, RT, Up, Up, Down, Left, RT, A

If it worked, you should hear a sound effect, be taken back to the chapter select menu, and have this spiffy little ribbon on your save file: