Should the amount enter in TurboTax for SSA income include the amount withheld for Medicare or is it just the net amount I received?

The total Social Security benefits you receive before any deduction for Medicare premiums paid.

The amount to enter is from box 5 of the SSA-1099 which you will receive in January 2021 for the 2020 benefits paid.

Is this your first year?  Do you have a copy of last year's?  Here a blank one to see.  You enter the amount in box 5 which is before deductions. Medicare deductions will be listed in the big box on the left side.

add the cash you received PLUS what was withheld for Medicare PLUS any voluntary tax withholding. THAT is your SSA income.

Thanks