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

So, I just need to settle our doctor's bill. Easy, right? Not so fast! The website says I need enter my Client ID, but where am mine? This whole thing is making me bonkers! I've checked everywhere, but it darn Client ID seems to be lost. It could be I just missed it? Could someone reach out to customer service?? Help! This is really a head