Form Personalization for populating terms date from a Contract PO in AP Invoice

The contract Po details gets populated in a descriptive field. I want to populate the terms date from the contract PO number on the basis of Invoice date or Invoice received as mentioned accordingly in Contract PO. The terms date should get populate in the AP_INVOICE_ALL terms date screen as and when the same contract is used while processing an invoice. I don't want to do any change in custom.pll.

Could somebody please guide?