Using "Update Document" to create new document in a Document Set

I am trying to use the "Update Document" action to create a new document within a document set. 

Details:

• I have a List and a Document Library (that contains a document set content type). 
• I am running the workflow on the list item, when it is created.
• When the list item is created, it goes and creates a document set in the document library (so I have the document set ID stored as a variable)
• When the workflow gets to a certian point, I would like to use the "Update Document" action to save a document to the document set the workflow created earlier.   

My question is, how do I tell the new document what document set it belongs to?  Is there a column that I would drop the document set ID into?

Any help is appreciated.

Alise