A quick glance at the Law and the Regulation seem to say "paid during the taxable year". However, IRS Publication 502 says:
"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."
https://www.irs.gov/publications/p502#en_US_2022_publink1000178852
To me, that seems to say it would not be on your 2023 return. But I'm not sure what authority that statement in the Publication was based on. But I haven't had a chance to really dig into the matter to research it.
