I am completing a NQSO transaction. I paid taxes not only through my employer (W2), but through my broker when the transaction occurred. Where do I enter this tax?

[TomYoung]

"I paid taxes not only through my employer (W2), but through my broker when the transaction occurred."

Is that really and truly the case?  If the broker withheld taxes then those taxes would show up on Boxes 4 and 16 of the 1099-B.  This is called "backup withholding" and is fairly rare.  If those two boxes are blank you CANNOT simply enter numbers in them.  The IRS will pick up on that error in a New York minute.

The broker may have told you, in some supplemental information, about shares "sold for taxes" but unless those dollars show up on the 1099-B, the cash raised by the sale of those shares was passed back to the employer, who paid the government(s) and included those amounts in the taxes withheld boxes of the W-2.

I think you are facing a different problem.

[LisaBr · Accepted solution]

If your tax paid is not included in your W-2, it should show up on your 1099-B, Line 4.  

On TurboTax, as you enter the 1099-B, click on I'll enter additional information on my own.  This is located at the bottom of the page.  You can add the tax paid here.

Since this is a NQSO transaction, you may need to enter more information about the exercise of this stock.  You will use the Guide Me, however, it will not let you enter the tax withheld since it expects this tax to be reported on your W-2. 

If this is the case, report the tax under Deductions and Credits, Estimates and other Taxes Paid, Other Income Taxes, go to the bottom of the next list and choose, Withholding not already entered on W-2 or 1099.