most special function keys, to the computer, arent actually a single key, but a combination of keys. Like the email button may be ctrl+alt+F8 or something like that, you'd just have to google it. If you found the combo for the key you want to assign it to, you can use openbox configuration, like this:
It is for screenshots, but it shows how to set up printscreen button to run an shell script when the printscreen button is pressed.
If it isnt smoking and blown into pieces, it's not pushed far enough yet.