I qualify for FICA Exemption, how do I add this to TurboTax?

When you enter her W-2C, you would enter her information as to how it was reported by her employer without changing anything.  If you are referring to the income tax imposed on her portion of the income, that is different than FICA.  If you owe taxes it is most likely from another item or computation on the return.

Who Is Exempt from Paying Social Security

However, the exemption isn’t automatic; you must apply for it. The exemption is unavailable if you’ve ever been eligible to receive benefits under the Social Security program regardless of whether you actually received the benefit or not.

To claim exempt, you must submit a W-4 Form. Do not complete lines 5 and 6. Enter “Exempt” on line 7. Note: You must submit a new W-4 Form by February 15 each year to continue your exemption.