The above is correct. The tax withheld should be listed in box 4 and 16 on your Form 1099-B. When you enter the investment sale in TurboTax, you will see an option to indicate that your Form 1099-B has additional boxes. You need to click on that box to see the box 4 and 16 where you can enter your backup withholding.
Also, since you are dealing with restricted stock units, when you enter the stock sale you need to be sure you include the vesting income in your cost basis, so the capital gain on sale of the stock to pay your taxes will be minimal. You will see an option when you enter the Form 1099-B to indicate that the basis reported on the Form 1099-B is wrong or missing. Choose that option and then enter the corrected cost basis.
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