The MyPost Business system doesn't have a way to create Return Labels via their API (what we use to connect to their system).
You can easily create return labels from within the MyPost Business website by following these steps:
Log into MyPost Business
Find the original consignment in the History section
Click View to expand the transaction
View again to expand that consignment
Click the Create Return Label button
Note: The label needs to have been generated and paid for before it will appear in the History section.