Sales order Copy functionality Customization

Hi Team

I have a requirement where i need to copy sales order and on the copied sales order the customer PO number should get wiped off (i was able to do this using personalization), however when i enter the customer PO number it dosent stick because of the same personalizaiton

Attached is the personalizatiion document for review any idea how do i achieve this? i.e. when i copy a sales order the copied order should not have customer PO information, however when i manually enter the customer PO number it should stick.

Thanks

Raghav