If you have already been using numbered purchase orders and wish to continue in the same sequence, double-click on the "No:" label (just left of the PO number field). Doing this will temporarily un-lock the field so that you can type in a starting number other than "000001". Make sure you type in a six-place number including leading zeros.
Save the order, and confirm that it shows up in the saved purchase orders list at the bottom of the screen.
From here on out your PO numbers will go up from the starting number, in line with your old system.
DO NOT use this feature regularly! It is only intended to be used ONCE, idealy when you first start using the system.