You don't. Those taxes have already been reported on your W-2 and you can't deduct them again. The cash raised from the sale of stock "for taxes" is passed back to your employer, who pays the government(s) and then scatters that among the various "taxes" boxes on your W-2.
If you are worrying about somehow not being able to enter the taxes paid with the cash raised from the stock sold "for taxes", then I know you are making the mistake of using the wrong basis to report the sale and seeing your tax liability go up when you do. The answer is to use the correct basis when reporting the sale.
Your correct per share basis is the same as the per share "fair market value" used by your employer to determine the compensation created by the vesting. That can be figured out, if you don't already know by the following equation:
(compensation created by the vesting of the RSU) divided by (GROSS number of shares that vested)
Enter the 1099-B exactly as it reads and then click the blue
"I'll enter additional info on my own" button. On the next page enter
the correct basis in the "Corrected cost basis" box. The correct basis is: (# of shares sold) x (per share basis for that lot.)