Thank you MiriamM,
"However, if you're using QuickBooks Desktop you'll need to have your own credential to be use in processing payments. This is a new authentication system for QuickBooks Desktop Payments users to make your payments more secure."
But I already do have a credential for processing payments. I use Quickbooks desktop, but to sign into payments (from within Quickbooks desktop, or through a web browser), I have to enter my quickbooks online password to connect to payments.
I'm a quickbooks proadvisor, and have QBO credentials for logging into my clients books. That same username and password is what I use to login to the quickbooks payments website.