You don't need to bill Medicare for a medication like Prolia. When Medicare processes a Part A claim, it pays the provider directly for the service.
With a Part B claim, who pays depends on whether the provider has accepted the assignment. If they have, Medicare covers 80% of the procedure cost, and you cover the remaining 20%.
It works similarly for Part C (Medicare Advantage) or Part D, except you'll have to pay out of pocket if the doctor doesn't accept the Medicare assignment.