Auto fill Approved by in PO number when PO number not available

Hello,

When order is placed without PO number there is no info in the in the Purchase Order Number field. When charge is approved an post this charge will be seen as a ticket. When you have a lot of hour tickets and just one charge it isn't easy to invoice that one charge. My request is when PO order is not filled in SB will auto fill in the text Approved by [name]