Welcome to QuickBooks Community, David.

I want to help you further with your question but I need more information of what exactly you need the PO Box for. Please add some more details in the comment so I can assist you further.

If you need to add the PO Box as the company's mailing address, QuickBooks will require the company's physical address. 

The system currently doesn't allow a PO Box in that field. You will get the error, Physical Address cannot be a PO Box message when the system detects a physical address isn't used. 

For additional information on how to change email or business info, you can check out this article: Update business and account information. 

I'll be waiting for the additional details so I can help you further. Mention me any time. Have a nice day!