If you are a North Dakota resident serving in an active or reserve component, the military pay that is taxable on your federal return is exempt from North Dakota income tax.
There is nothing to apply for. Leave your military pay out of the income you report on your North Dakota return. If North Dakota tax was withheld from it by mistake, file a return to claim the refund.
Your Form W-2 and Leave and Earnings Statements showing the military pay, kept with your tax records.