MB_MIGO_BADI: Fields on Header - Accessing PO number

Hi Experts,

I am using MB_MIGO_BADI to add custom fields to the MIGO header tab. Inside the BADI methods/function modules, I'm not able to access the PO number. The PO number that is entered on the MIGO screen is needed to fill the new fields that I've created. How do I get the PO number?