That's a typical setup, locally and in few other places dealers have specific trucks for this called high lifts:
There's chute that comes off the back, it can also be used at a right angle. Back the truck up, lift it so the chute has enough angle and dump it in.
If you're doing it with a pickup you could do the same thing but you'll have to shovel the coal onto the chute. The chute will need at least a 40 degree angle so you will have to be fairly close. On the cheap you could make one with a little bit less that the width of the window and vinly sheeting the thickness of siding. Tack some furring strips to the sides. You could also use any other slippery surface. Coal trucks have custom aluminum chutes, very expensive.