Not possible in normal warcraft. What you can do is:
Detect arrowkey clicks (left, right, up, down)
Detect escapekey ("player skips cinematic" event)
Or... Give a unit an ability with a certain character as hotkey. Whenever the key is pressed, the ability will be cast and you'll detect if the letter was pressed.
Or... use RTC (check hosted project) if you know jass.