You will have the opportunity to enter taxes withheld in the stock sales section (see screenshot). To get there, go to:
Federal Taxes > Wages & Income >
Scroll down to Investment Income, then click Show More
Select Start next to Stocks, Mutual Funds, Bonds, Other
Answer Yes to "Did you get a 1099-B or a brokerage statement?"
To import your 1099-B, select your financial institution and Continue (there have been integration issues, however)
To enter it manually, scroll down from the bank logos and select I'll type in in myself
