Autopopulate Requisition/PO Header DFF from requester

Hi

We have created a 'Bill-to email' DFF at Requisition and PO Header level. That is a free text field where user an enter the email. However the requirement is to initially default it from requester email ID. Can you suggest way to achieve this.

There is an option at DFF setup level to default, but not sure how that can help here

thanks