Has anyone successfully implemented the OS Maps API in Portal for ArcGIS? We're using 10.4, but had the same issue in 10.3.1 too. I received a key registering at https://www.ordnancesurvey.co.uk/forms/os-maps-api-signup and followed instructions for adding to Portal as per the ArcGIS Online instructions at https://apidocs.os.uk/docs/os-maps-guide-1 . Obviously Portal and AGOL are not the same, but both ESRI and Ordnance Survey assure me this same procedure should work for Portal. However, I get an error saying "The WMTS service can't be added to the map. It's either not available or you have entered an invalid URL for the type of layer you want to reference." The URL is fine as I can add it to AGOL and QGIS, just not Portal - therefore our firewall is presumably OK too. We've run all sorts of Wireshark and Fiddler traces but to no avail. Chrome shows a message with 'No 'Access-Control-Allow-Origin' header is present on the requested resource' but I don't know what this means or if it is a red herring. Any help appreciated.