Thanks George, and yes, I've read this, but based on that I would expect my setup to work, since I'm using IWA and have same security settings for the parent map service as the WMS. I do wonder if it is different for Stand Alone servers as opposed to Federated ones. I do see this in the devtools in the browser, strange since Portal and my federated server should trust each other: Access to XMLHttpRequest at 'https://federatedserver.domain.com/arcgis/services/test/MyMapService/MapServer/WMSServer?SERVICE=WMS&REQUEST=GetCapabilities' from origin 'https://Portal.domain.com' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.
... View more