Hal_Al
Level 15

Get your taxes done using TurboTax

It appears to be a short coming in TT.  I could not get it give me that option either.

 

I have the download software, the adjustment can be made in the forms mode.  Forms mode is not available in the online programs. 

 

One alternative workaround (I've seen it  recommended, in this forum) is to just enter the 1099-S amount as your cost basis, resulting in 0 gain.