Place the auger on the ground, put the tractor in neutral and let the clutch out fully. As long as the tractor is in neutral it doesnt matter.
Keep foot on brake to keep tractor from moving
You will probably have to jig the tractor forward or reverse a few times while drilling. When you start out with PHD its best to stop the pto and adjust tractor, then restart drilling.
Ive been drilling holes for many years, and i used to simply depress clutch 1/2 way, set gear while in low range, and slowly release clutch WHILE i continued to drill. dont try this till you comfortable with your setup.
With my new HST tractor, moving while drilling is a piece of cake.