How Do I Add A Deduction Code to the W-2 Tax Form Definition?

Hello,

I have recently created new benefits plans effective 12/01/2020. Since there will be a deduction in December I need that to be represented under Box 12DD in our W-2. When I try to update our tax form definitions, the deduction code is not available for selection. Does the payroll need to be confirmed before this deduction code is accessible? Or did I miss a step in my set-up?

HCM 9.2 PUM 36

Thanks!