To use a probe input for G38 probing moves you first have to hook up your probe to motion.probe-input. In this example we will use the parallel port input pin 15. In your .hal file add a line like the following making sure you pick the pin you have hooked your probe to.
net probe-in motion.probe-input ⇐ parport.0.pin-15-in
For more info on probing see [sub:G38.2:-Straight-Probe] and [cap:Probing-codes]