When you go through the medical expenses section of TurboTax, you'll reach a page that asks How much did you spend on insurance premiums? Add the amounts you paid for Medigap and prescription insurance (not shown on Form SSA-1099) and enter this total in the Medical insurance premiums box. (Click the screenshot below to enlarge it for reference.)
The FAQ below tells you how to get to the medical expenses section in TurboTax.
https://ttlc.intuit.com/replies/4774888
Premiums for Medicare supplemental insurance and prescription insurance are deductible, if they were paid for with after-tax dollars (usually are). Also out-of-pocket prescription expenses are deductible (since "out of pocket" usually means after-tax).
However, note that to actually affect your tax return, only the medical expenses that exceed 7.5% of your adjusted gross income (AGI) are reported on Schedule A (Itemized Deductions). And if your Itemized Deductions are less than your Standard Deductions, TurboTax will take the Standard Deduction anyway.
But until you enter all your deductions, you won't know, so go ahead and enter the items under Federal->Deductions & Credits->Medical.
[edited 2/15/21 | 12;07 pst]