You can enter ALL of your health and dental insurance premiums so long as they weren't withheld on a pre-tax basis or reimbursed in 2018. This includes all supplemental health insurance, such as Medicare B supplemental medical insurance, Medicare D voluntary prescription drug insurance, and long-term nursing home coverage premiums. Do NOT include any "Medicare Tax Withheld" on your W-2 Forms (W2).
Note that the deduction is NOT available to any individual who paid these health insurance premiums on a pre-tax basis. Pre-tax means to subtract such things as health insurance premiums from your gross wages before withholding taxes are computed. Health insurance premiums are typically deducted from wages on a pretax basis. See your payroll department regarding pre-tax information.