The names of the HID keyboard scan codes as found in header files such as key_defs_keyboard.h are slightly confusing. They are named according to what they would produce in your OS if your OS was configured to use US-QWERTY. But what the HID scan codes mean is more along the lines of “the physical key that is found on the second row, third from the left on an IBM AT keyboard”. But that’s not very human-friendly, and since most people have a QWERTY keyboard in full view when they program, it makes sense to name the physical keys according to what is printed on them.
If you want a dead-diaeresis, under normal circumstances you would tell your OS to use a mapping that includes dead-diaeresis as one of the outputs. This could be a keymap (such as US-international+dead_keys), an input method (such as compose key), or some other method that requires synchronisation between the keyboard firmware and a novel OS driver.