I am trying to connect Survey123 to our new Enterprise Portal. Our Portal is federated with ArcGIS Server and our security is managed by AD. I can successfully get to the Survey123 site using our portal url (https://survey123.arcgis.com/?portalUrl=https://exampleportal.esri.com/arcgis) but when I click 'Sign In' I get a 404 error:

I have gone to my ArcGIS Server Administrator Directory and added survey123.arcgis.com as an AllowedOrigin

Any ideas what could be going on here?