That goes in box 12. Pick DD for the code and enter the amount. It doesn't matter which box 12 you use. The lowercase a-d are just part of the box label. You don't enter them. You only enter it from the W2, nowhere else.
DD means cost of employer covered healthcare. The codes are listed on the back of the W2. Or see the W2 here, codes start on page 8
https://www.irs.gov/pub/irs-pdf/fw2.pdf
A 457 Plan should be in box 12 with code G. Do you have that one? After you enter the first box 12 it will expand so you can enter another box 12.
Q. I have a code DD deduction on my W-2. What does this mean?
A. Code DD includes cost of employer-sponsored health coverage. Information only.
Q. Where do I report it?
A. You enter your W-2 exactly as received (code DD and the amount in box 12). TurboTax will automatically report it, as needed. In the case of code DD, it's only informational and nothing about it is entered on the tax forms. Employer paid health insurance is a tax free benefit.
Reference: https://www.hrblock.com/tax-center/irs/forms/understanding-form-w2-box-12-codes/