Differential pay is the gap between your military pay and your civilian wages that your employer makes up while you serve in a reserve component. Pennsylvania normally taxes it, but it is not taxed while you are serving on active duty in a combat zone or hazardous duty area.
There is nothing to apply for. Leave the differential pay you received during combat zone or hazardous duty service off the income you report on your PA-40 return.
Your employer pay records showing the differential pay, and orders showing the combat zone or hazardous duty area service dates.