how to add payment term in po approved workflow notification header

 I am trying to add payment term in header of the notification of PO has been approved notification of PO Approval Workflow.

I have created a attribute of name Payment_terms of Document type and in Value field i am giving procedures name which gives output as payment term.but i am unable to get it.i have also attach it in message.

Default value : Plsql:XX_PAYMENT_TERM/&PO_TERMS