The original answer didn't address where to enter Medicare payments not paid out of Social Security benefits because that wasn't the question that was asked.
Whatever portion is shown in Medicare Premiums boxes of the SSA-1099 box is automatically included on Schedule A. The portion paid separately would be entered under Deductions & Credits -> Medical -> Medical Expenses -> How much did you spend on insurance premiums? (On Schedule A there is no breakdown of the various types of medical expenses, they are all lumped together, so you can generally enter any type of medical expense in any type of the TurboTax box that asking for medical expenses. The breakdown is for your personal recordkeeping.)
