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
