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

So, we just need to settle the doctor's bill. Easy, right? Not so fast! The website says I have to enter my Client ID, but where is mine? This whole situation is making me nuts! I've checked everywhere, but it darn Client ID seems to be lost. It could be I just missed it? Should you contact customer service?? Help! This is really a headach