Add a PO number to an Invoice

You can create a webform that's assigned to any corporate event type. Then just add a field to that of PO number and click the check box to make it a variable. When the customer fills it out it'll allow the PO number to be a variable.

Then in the settings for invoices, you can add the PO variable from the webform to the header or footer.

Go to Configuration > Webform > Add a Webform. Kindly refer to the video tutorial below.