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For anybody having issues with CORS config on their IIS arcgis web adaptor, here's what I added to the web.config under wwwroot\arcgis-
I added the following under the <system.webServer> section-
<!--<add name="Access-Control-Allow-Origin" value="*" />-->
<add name="Access-Control-Allow-Methods" value="GET,PUT,POST,DELETE,OPTIONS" />
<add name="Access-Control-Allow-Headers" value="Content-Type" />
The request header for Access-Control-Allow-Origin was already defined someplace, I could not figure out where, so that part I left commented out. Only needed everything below that.