I just went through this same scenario and here's what I did. When I exercised/sold the options, taxes were taken out before the net proceeds were released to me. From the gross sale amount, the cost of the options and taxes were taken out and I was left with the balance. I was able to load in the 1099B from Schwab but it showed no taxes paid. I spoke with them and when I was on hold there was a message saying I had to adjust the cost basis manually. I spoke with them and they sorta kinda told me what to do in a "im not a tax avdisor" way. I also spoke with a mostly helpful turbotax help desk person who verified this.
The problem for me is that all the info I needed was scattered. I had the 1099-B and then I had another form from them that showed the taxes paid. I put it all in a spreadsheet to have it all in one place and make sure that the info from each batch of transactions lined up. On the worksheet for the 8949 form, you have to override the cost (1e) with the exact amount of the proceeds (1d), which makes the gains $0. You can also enter the federal and state taxes paid. There is a worksheet for the 8949 form that steps you through this on about 2-3 pages of prompts and then another 1-2 page of questions.
Hope this helps!
