This is not supported in App Service.
You can up-vote this feature request in Uservoice: https://feedback.azure.com/forums/169385-web-apps/suggestions/9912714-enable-users-to-create-custom-error-pages-for-403
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I was following along with this tutorial
But my custom error page isn't being displayed. This tutorial is also over 10 years old so I wasn't sure if settings need to be changed.
I run the app, and go to a page that doesn't exist, but it doesn't show the custom error page.
Is it possible to have custom error pages deployed on Azure App Service (ASP.NET Web Forms Application)?
This is not supported in App Service.
You can up-vote this feature request in Uservoice: https://feedback.azure.com/forums/169385-web-apps/suggestions/9912714-enable-users-to-create-custom-error-pages-for-403