Do you mean that you have to check box B at the top of Form 8949, or do you have to enter adjustment code B in column (f) on Form 8949? Checking box B at the top of Form 8949 indicates that the sales reported on that page are short-term and the basis was not reported to the IRS. Adjustment code B in column (f) on Form 8949 indicates that the basis shown on the 1099-B is incorrect and you are making an adjustment to make the basis correct.
Entering investment sales in forms mode is very complicated and error-prone. That's why TurboTax strongly recommends using the Step-by-Step interview to enter investment sales.
If you need to enter sales with box B checked at the top of Form 8949, for the "Sales section" at the top of the screen where you enter the details of the sale, select "Short-term basis not reported to IRS (noncovered)."
If you need to enter adjustment code B in column (f) on Form 8949, on the screen where you enter the details of the sale, enter the basis that is shown on the 1099-B. Below the basis check the box that says "The cost basis is incorrect or missing on my 1099-B." A couple of screens later you will be able to enter the correct basis. TurboTax will enter the sale on Form 8949 with code B in column (f) and the appropriate adjustment to correct the basis.
