Hey there, Martinh.
Thanks for following up on this thread. I'd like to provide some additional info.
At this time, adding the purchase order number is only available for invoices. I can see how it would be helpful to have this option for the Customer Payment screen. I'll be sure to pass your feedback along to the Product Development Team for review. Many product updates come directly from great user suggestions like yours. You can keep up with product updates by viewing the official QuickBooks Blog.
Please feel free to reach back out if you have any other questions.