I was able to resolve this Issue!!!  You can't create Custom POs, so it's tied to the "Standard" Template.  It seems like it's only for Invoices, but it's not!

Click on Gear and then under Company click Custom Form Styles

Under Action, click Edit to edit the "Standard" Template - this is tied to POs

Click the Content Tab near the top left

On the right side, click the top rectangle box to edit (this is where the PO # will print)

On the left under Forms, click the box for Form Numbers

This allowed the PO # to print on the Purchase order going forward.