As you can see looking at the "official" 1099-B:
https://www.irs.gov/pub/irs-prior/f1099b--2016.pdf
Box 1g identifies itself as a "wash sale loss disallowed."
If you enter an amount from Box 1g in the 1099-B interview TurboTax enters the amount in column 1g of Form 8949 with a code of "W" in column 1f.
Tom Young
