You can only enter the medical expenses you have actually PAID--not the outstanding amounts.
MEDICAL EXPENSES
The medical expense deduction has to meet a rather large threshold before it can affect your return. The amount of medical (including dental, vision, etc.) expenses that will count toward itemization is the amount that is OVER 7.5% of your adjusted gross income. You should only enter the amount that you paid in 2019—do not include any amounts that were covered by insurance or that are still outstanding. Of course, your medical expenses plus your other itemized deductions still have to exceed your standard deduction before you will see a difference in your tax due or refund.
To enter your medical expenses go to Federal>Deductions and Credits>Medical>Medical Expense
One question, though--- you said you will be making payments for a long time? Did you put the medical bill on a credit card? Or are you making payments to the hospital, etc.? If you put it all on a credit card you can enter the amount you put on the credit card.