Hi shikma!
Currently, we're unable to customize the Bill transaction page. However, you have an option to show the PO that relates to your bill. I'm happy to share it with you.
What you can do is add the PO number to the description field of a line item. If you're creating a bill for multiple purchase orders, just enter the number of each PO to the corresponding items on your bill.
If you have other questions about your bills, just leave comment below.
Have a good day!