Do not try to make the entry in forms mode. Entering investment sales in forms mode is very complicated and error-prone. And you can't make entries directly on Form 8949, even in forms mode.
You should be using TurboTax Premier or higher. In the Step-by-Step interview, on the screen where you enter the information about the sale, enter all the information, then click the button that says "I'll enter additional info on my own." On the next screen, enter the disallowed amount in box 1g, even though it's not on your 1099-B. Then click Done. This will fill out Form 8949 with the correct information, including code W, which is the correct code for a wash sale, not code B.
If you click the blue link for box 1g, you will see that the TurboTax help says the following.
"If the amount reported by the financial institution is not correct, or they have not reported a disallowed wash sale amount even though it applies, you should enter the correct amount of disallowed wash sale amount here."
This is also what the IRS instructions for Form 8949 say to do.
