It works almost the same as the self-propelled one. Park it with it's right side to the troughs and keep it hooked to a tractor. Then use a FEL to dump one bale of straw in the bucket. Switch to your tractor again and turn the Primor on (look for the unload-pictogram). That should do the trick. To my knowledge it doesn't matter or not whether the door is open, but loading with the door open is a little easier.
On the FEL, I use the agrovector, because it has that nice extendable boom. Makes it a lot easier to dump bales in; just stab one bale, drive it to the Primor, extend the boom as far as you can and maneuver the fork above the bucket. Tip forward and there you go.