If the broker is reporting the basis to the IRS then you enter that number in TurboTax's default "1099-B" entry form, and then you "correct" it. After entering the 1099-B exactly as it reads click on the "I'll enter additional info on my own" blue button. On the next page enter the correct basis in the "Corrected cost basis" box. The correct basis is, most likely, that "adjusted cost" reported on the Supplemental information.
TurboTax will report the sale on Form 8949 "as reported by the broker" but will put an adjustment figure into column (g) of the Form, a code "B" into column (f) of the Form, and the correct amount of gain or loss which includes the adjustment.
Tom Young
NOTE: TURBOTAX CHANGES THE SECURITY SALES INTERVIEW JUST ABOUT EVERY SINGLE YEAR. THE INSTRUCTIONS ON "HOW TO FIX" THE BASIS REFLECT THE STATE OF THE INTERVIEW FOR TAX YEARS 2016 AND 2017.
