In MIGO populate the header data based on line item value

Hi,

I have a custom defined header tab in the header of MIGO transaction in addition to the standard tab and it contains 5 input fields. And i have a custom defined item column in line item table just below the header. My requirement is : while selecting the PO for Goods receipt and return delivery, the one of the cutom defined field (say: "status" )in header has to be populated with some value based on the custom defined line item(say : "flag" ) value of that PO.

Please help me out how to populate the header value based on the line item value while loading the PO?

Thanks in advance.