Since you arleady know what imputed income is, there's no need to refer you to https://www.patriotsoftware.com/payroll/training/blog/a-closer-look-at-imputed-income/ (I put it here for others reading this thread).
Your employer has already done the math and you don't have to do anything. It's already included in box 1 of your W-2. Do note that since it's for health insurance, the insurance costs can be included as a medical expense for inclusion on SCH A. Of course, until the total of all of your medical expenses exceed 2% of your AGI, they won't make any difference to your tax liability.