While playing the game (free or story mode), just start typing on the keyboard (there is no codes menu or input field).
backspace > right shift > whatever letter should be after ...
The key's are typed, no need to hold them down. Also there are no big letters or combo key to keep something down (like shift + letter).
Some codes (not the ones with backspace) might require to set the keyboard to US type (in windows keyboard settings).
Side note : please don't write codes in comment section, if the issue persist contact me via email nymphokyun@gmail.com