Oder mit _IsPressed()
[autoit]
#Include <Misc.au3>
$countdowntime = 15000 ; z.B. durch Userabfrage
SplashTextOn("Countdown", $countdowntime/1000 & " sec")
For $i = $countdowntime To 0 Step -1000
SplashTextOn("Countdown", $i/1000 & " sec")
If _IsPressed("78") Then Exit; F9
Sleep(1000)
Next
SplashOff()