On the screen where you enter your sales transaction details, there is a check box for: "I have other boxes on my 1099-B to enter". Checking this box will reveal more entry options, one of which is "Federal Income Tax Withheld". See screenshot below. If you don't want tax withheld, you can contact your broker to see about what forms need to be filed so that your aren't subject to tax withholding.
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The answer is incorrect. If you are entering a summary of sales for each category, you can click "I need to enter amounts from box 1f, box 1g or other adjustments" but there is no place to enter tax withheld. Where can I enter tax withheld?