You can customize the purchase order number, user71610.
There's an option for you to that. However, you'll need to manually add your own transaction number every time you create PO. If left blank, QuickBooks Onlline automatically assigns the number for you.
On the Purchase Order window, click the Gear icon.
Check the box for Custom transaction numbers, then click X to close.
You'll now see the PO no. box, enter desired number.