StockTransfer BPLId setter

Hello, experts!
I looked everywhere, but I can't seem to find how SAP B1 DI API sets the BPLId variable. It's readonly on StockTransfer object.

This is a brazilian issue because of how we use branches (OPBL instead of OUBR table). Any help would be appreciated although I confess is pretty frustrating working with SAP due to this lack of documentation.