MuzammilAK-7543, As TakahitoIwasa mentioned it depends on the Host header, and the Fortinet document that was shared may provide some pointers.
In typical scenarios, just to highlight on how it works with Application Gateway-
Case 1: default domain ( ".azurewebsites")
When App Service does not have a custom domain associated with it, the host header on the incoming request on the web application will need to be set to the default domain, suffixed with ".azurewebsites.net" or else the platform will not be able to properly route the request.
The host header in the original request received by the Application Gateway will be different from the host name of the backend App Service.
Case 2: In this case, you are required to have a custom domain (and associated certificate) available to avoid having to rely on the default ".azurewebsites" domain.
By associating the same domain name to both Application Gateway and App Service in the backend pool, the request flow does not need to override the host name. The backend web application will see the original host as was used by the client.
Kindly check this doc for additional info: Zero to Hero with App Service, Part 6: Securing your web app
Configure App Service with Application Gateway