App Service now supports to define custom error page for 403, 502 and 503 error codes.
This capability is available for both Linux and Windows workloads when using a Premium SKU.
NOTE this is not available for ASE v2 SKU
Before setting up your custom error page, you need to create the new HTML page which must be less than 10 Kb.
Once you are ready, connect to your Azure portal (https://portal.azure.com/) to locate the web application you want to configure custom error page
Access the Configuration blade to go to the Error pages section to edit the error codes
If you edit the 403 error page, you can test it by editing Access restriction in the Networking blade to block your IP address