I am still trying to figure out why you want this on the Sch C at all ... if you enter 1099-g in the search box you can jump directly to where you can enter that form & withholding.
If you insist that it needs to go on the Sch C then the withholding is entered here ...
1. Federal Taxes
2. Deductions & Credits
3. Scroll down to Estimates and Other taxes paid
4. Select Other Income Taxes, select Withholding not already entered on a W2 or 1099 (or whatever other option applies to you)
