You can't. You have to design your own form to use instead of the InputBox.
Open it using the acDialog option. This causes the calling code to pause until the
form is closed.
Include a Cancel button that closes the form and an OK button that only hides it.
Either one of these cause your calling code to resume running.
You can then test to see if the form is still open (though hidden). if it is you
know they pressed [OK] and you can pull the entry they made from the form and then
close it. If the form is not open then you know they pressed [Cancel] and can drop
out of the routine.