Where to enter a Customer PO value for Sales Invoicing (to create burn reports)?

SOLVED

Hi there

Looking for some guidance if it's possible to enter the value of the customer provided PO when sales invoicing is created to help in the creation of burn reports?  I don't actually do the billing, but just trying to dig around here on misc screens, and keep coming to dead ends.

As it is now, we capture the customer PO# on our sales invoices, but would be extremely to house the value somewhere as well for reporting purposes.

Thank you!