SETTLE MY MEDICAL BILL: LOCATE MY PATIENT ID

I recently visited/saw/met with my doctor and now I need to settle/pay/clear the bill. The online portal is asking for a client/patient/account ID, but I can't seem to find/locate/remember it. Where can I check/look/search for this number? I've already/previously/recently looked/searched/checked my medical records and statements but no luck! Is