If you want to do text entry, you can use winwin_keyboard_get_string - no need to map every key to a character by hand.
Also, this is somewhat unrelated, but there doesn’t appear to be an option to restrict fullscreening another window with Alt+Enter.
That’s weird, GameMaker is definitely not aware of additional windows so I guess Windows applies fullscreen mode to whatever window that’s currently active? You can disable that in Game Options ➜ Windows ➜ Graphics and handle switching yourself via keyboard_check(vk_alt) && keyboard_check_pressed(vk_enter)
.