defaulting header texts in ME21n texts tab

Hi Experts

I got a requirement where the shipping type texts in Header Texts tab should be defaulted in me21n transaction

When user open me21n transaction and enter the details like order type vendor number , material number po quantity plant storage

and hit enter

then automatically the Shipping Item text in Texts tab of PO header tab should be defaulted with the value belonging with the Shipping Item.

Shipping Item is obtained from route table

so basing on route value shipping Item should be determined  and after that get the text of the shipping Item

I need an user exit /Badi where we can achieve this functionality

Thanks

Pawan Akella