Hi,
As Mr. Mukesh already suggested, transaction variant is the best option. You may create the variant in SHD0 transaction. While creating the variant, you may enter the plant as 1000 and movement type as 201 and then maintain these fields as "with content" in the transaction variant as shown below:
Activate the variant and test the MB21 transaction again. System will default these fields. The variant can be activated at transaction level or even user level. You may check / search in SCN for the same.
Regards,
AKPT