When you create a new invoice, or open a previously-submitted invoice, that is associated with a PO, you will see multiple tabs on the top-left of the invoice header.

Select the “Purchase Order” tab and you will see a section called “Purchase Order Details” on the left-side of the pane. Here you can see the Start Date, End Date, Currency and PO Description:

In the “Itemization Summary” you will see one or more lines for the PO which show the GL Account used for setting up the PO  e.g. “62510  Consultant Fees”.

You should use the same GL Accounts when billing invoices against the PO.