PAY MY DOCTOR: WHERE'S MY CLIENT ID?

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 proceed with the payment. Can someone assist? I looked over my statements from the doctor's office, but I can't find it there. Is there a alternative method to find my client ID? {I'm desperate|I urgently requir