To quickly add your PO Number, we recommend including it in the "Job Description" field as pictured below before printing your advance invoice:
Another option is to make custom label text and call it "PO Number". When you make a new custom field label text in settings, the new field(s) will only appear in subsequent jobs you create in that personality and not in the current job you want your custom label with PO number to appear. The best way to access your new custom label field in an active job is to duplicate that job for which you want custom labels. To make custom labels, go to Settings > Document Appearance > Label & Section Text.
Please follow this link to our video tutorial on duplicating jobs and let us know if this helps or if you have further questions.