Interesting resolution here, as the Microsoft documentation states to enter the https://dev.crm.dynamics.com/ORGANIZATIONNAME. It turns out this is NOT the actual name of your organization but the GUID!
After that, we were successfully able to use the CRM Online Disovery Service. Also of note, CRMOnline has two different discovery end points, one for CRMOnline above, and one for Office365 below. Make sure that you identify where the customer’s CRMOnline is hosted.
Office 365 discovery url: https://disco.crm.dynamics.com/ORGANIZATIONNAME