My wife and I both had FIT deducted from our social security income. Where can I enter voluntary federal income tax withheld? There's no box on the TurboTax form.

TurboTax has a box to enter your social security federal tax directly under the box to enter your social security income.  See the screen shot below.

Withholding federal taxes on your social security benefits is voluntary.  So any voluntary federal tax withheld on your social security income is the amount you need to enter into TurboTax.