a BADi has more to do with ABAP than with MM. From your asking about elaboration it is already clear that you are not familiar with ABAP at all, so describe in detail what you want and ask an ABAPer to realize it.
If the ABAPer has a little MM knowledge then you may get it back with the comment that it does not make sense to put item related information into a header text.
Just ask yourself how it shall be if you have hundreds of PO items. Hence you would need to think further and probably restrict the PO to a single item. If that is really liked? I doubt.