When a ArcGIS Server service secured via web-tier authentication is added to a web map widget in a Portal for ArcGIS dashboard, a console error stating "Access to XMLHttpRequest at 'https://server.domain.com/webadaptor/rest/info?f=json' from origin 'https://server.domain.com' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource." is returned.
Requests sent through a web server, reverse proxy, or load balancer that does not have any restriction of hosts allowed to make CORS requests appear to be allowed from hosts denied by the AllowedOrigins property. Configure the web server, reverse proxy, or load balancer to follow the same restrictions as the ArcGIS Server site:
https://enterprise.arcgis.com/en/server/latest/administer/windows/restricting-cross-domain-requests-to-arcgis-server.htm
After installing the CORS modules for IIS 10 and configuring the system.webServer, the CORS header error is no longer reproduced:
https://docs.microsoft.com/en-us/iis/extensions/cors-module/cors-module-configuration-reference#cors-configuration
https://www.iis.net/downloads/microsoft/iis-cors-module