PAY MY DOCTOR: WHERE'S MY CLIENT ID?

Alright, so I need to finalize my statement with Dr. Smith. I went to their website and it's asking for a account ID number. Problem is, I have absolutely no idea where that is. I've scoured my emails, medical records, but nothing! Could someone please tell me? Is there a help section I can reach out to? Perhaps on my last bill? Simplify Payin