Your federal withholdings will automatically be added to your return when you enter the income source. For example, if you have a W-2, federal withholding is reported in Box 2. When you enter all the information from your W-2 into TurboTax, the credit will automatically populated on your tax return, Form 1040, Line 25. The same is true no matter where the tax was withheld from - a W-2, 1099, or another source.
