If the additional amount that was withheld is not located in box 6 of your SSA-1099, you would report the additional taxes paid by following the steps below.

1. Select Federal Taxes 
2. Click Deductions & Credits. 
3. Click "I'll choose what I work on" 
4. Scroll down to Estimates and Other Taxes Paid
5. Click "Estimates"
6. Scroll down to "Other Income Taxes Paid in 2015"
7. Click Start next to "Withholding not already entered on a W-2 or 1099"
8. Follow Prompts