PAY|MY|YOUR DOCTOR CLIENT ID: WHERE'S MINE?

So, I just need to clear the doctor's bill. Easy, right? Not so fast! The website says I must enter my Client ID, but where can I find mine? This whole situation is getting me bonkers! I've searched everywhere, but this darn Client ID looks to be missing. Maybe I just missed it? Should I contact customer service?? Help! This is truly a hea