CHECK MY BILL: WHERE'S MY CLIENT NUMBER?

I've just gone to pay my medical bill online, but I can't seem to find my client ID. The portal is asking for it, and without it, I can't submit the payment. Can someone assist? I looked over my email from the doctor's office, but I can't find it there. Is there a another option to access my client ID? {I'm desperate|I urgently require to pay this