Problem Statement:
Trying to default subsidiary in sales order via workflow, for specific transaction forms only
Solution:
As per checking, it is standard behavior that the Subsidiary field is not available in the Set Field Action for your workflow. There is currently an enhancement for this:
314838 WorkFlow > Transfer Order record > Set Field Value > Subsidiary field is not available in the list of field
This can be achieved by Script but still, it is not advisable because NetSuite offers standard sourcing on the Subsidiary field upon selecting a customer.