If I may. Well, I guess I always do.

JBS, easy on me.
I think that having a preferred pharmacy anywhere in AC is non-productive. This could be filled out at the time of starting the practice and be completely out of date with what's in ePrescribe.
Now, if you are saying that the preferred pharmacy would automatically populate the script writer, that would be great, but I doubt that is possible.
For me, it is easier to save that step at the front end (where your receptionist puts that in -- giving one more step for error) and use it in the room and simply ask the patient what pharmacy they choose. Consider that as the time they choose their preferred pharmacy. It will stay there for the next time and next time...