Any withholding is in box 4, that's the total amount reported. The withholding is only a backup amount and the actual tax due is calculated on your tax return. You likely owe much more than 10% since the withdrawal is also taxed as regular income. Enter the amount from box 4 along with the rest of the information, and Turbotax will calculate your overall tax, penalty, give you credit for the withholding, and determine your remaining tax due.
If no withholding is in box 4, you need a corrected 1099-R from the plan trustee. If you report tax withheld without it showing on the form, the IRS will audit you for the mis-matched information.
