I received a letter from my Employee Health Plan that states I received gift cards as a reward for participating in a wellness program. Where do I list this as additional income as the letter states?

I received $200 in rewards from an employee wellness program that is part of my employee health plan (Humana  Vitality).  The letter I received says that this should be counted as income and that it was not included on my W-2.  I'm wondering how to account for this additional income?