Where do I input the cheat codes?
The Panther Witch, Zen.
After scaring her at the start of the game in the witch's library: Get the learn from history set to get leftover cookies. Return to the library and lure her out with them. Attempt to visit her again and there's an input box which doubles as a way to put in cheat codes. (The input box remains even if you clear her quest with the expected answer you learn from Ranko)