Change default storage location during creation of PO

Hello all,

I have the following question.

When creating a purchase order using ME21N and filling in the items, the storage location is retreived from the material masterdata and displayed as default.

I need to change this default, because the storage location must be determined in another way.

So when entering the values for the item in the screen, another Stloc needs to be determined and displayed, overwriting the Stloc from the material master data.

For this I probably need a BAPI.

Can anyone tell me which BAPI to use and how I can implement this?

Because I also no experience with BAPI's.

Thanks in advance.

Kind regards,

Marco