BPM to Capture the the QuoteQty.CalcUnitCost field via Button

Hi Everyone,
I’m looking to capture and store quoteqty.calcunitcost in a UD field from the quote work sheet
I would execute the code through a button event.

I looked at a BPM and noticed the calc field was not availale for selection.
How else could I capture and store this calc field?

Thank You

Don