f-29 document header

hi guru's,

there is requirement from the client that while make the entry for down payment from customer thru Tcode : F-29, Sales order number should be obligatory.

currently sales order filed is not appearing any where in the document. please help to make the " sales oder number" is required filed while entry the transaction. I tried by making the sales order number required filed in OBC4 but it's not appearing in the document.

it's urgent requiremnt. please help on priority.suitable answers will be rewarded.

thanks

yatheendra nath