Place the first page of your tax form before you. The top portion is for filling out your personal information. Locate the boxes for your name and Social Security number and fill them in, placing your last name, first name and middle initial in the appropriate places.
Locate the boxes directly beneath this. There is space for your address. Add your P.O. box to the "Address" line. If you are using an electronic version, enter the P.O. box in the "Address" section. Enter it in this fashion. "P.O. Box" followed by the number.
Enter the city, state and zip codes in the appropriate boxes beneath this. You are finished adding your P.O. box for your tax return.