Behind the clutch pedal, up towards the top, there should be a safety button that controls that function. Every time you push down the pedal far enough, the switch "allows" power to the starter.I suppose if you figured out a way to keep the switch ON, you'd be in business.
If you do figure out a route for that, BE CAREFUL!!!! The starter is MORE than strong enough to power the truck forward if the pedal is not depressed.
That's why that switch is there. May hit the car or object parked in front of you, or it might send you into oncoming traffic, thats how my great-aunt died.