So I assume that you already know the rules for what will determine the subsidiary.

Ideally, you’ll be pulling that information in from another record referenced on the WO. I image the most likely candidates would be the Assembly number or from the employee record of the creator, or the location.

This should be pretty simple though we don’t use subsidiaries so I could be missing something here.

When you create the WF, the record type should be “Transaction” and the Sub-record should be “Work Order”. Sounds to me like you only need this On Create.

Create a new Action “Set Field Value”

Now what you use as a trigger is going to depend on what you’re sourcing from and what you have happing automatically in the record. If for example certain employees have a default location and that’s automatically populated upon entry then I’d trigger on “After Field Sourcing”. If you’re pulling it from the assembly number then you’ll want to do “After Field Edit”, you could do that for location as well if they input it. Select the pertinent field, and for the Value you want “From Field”, then reference the value.

That should literally be it.  Without more details on where the subsidiary information is coming from I really don’t have more.