Since the medical services were actually performed in 2019, you would claim the expenses on your tax year 2019 return, even though they were paid for ahead of time.
According to
"What Expenses Can You Include This Year?" in IRS Publication 502, Medical
and Dental Expenses, "You can include only the medical and dental expenses
you paid this year, but generally not payments for medical or dental care you
will receive in a future year."
