@SLIMANI Smail Thanks for your response. It is recommended to have custom domain when integrating the application gateway services with multi-tenant azure services like App service, Azure Spring apps etc., we want to make sure that the App Service is listening for the appropriate hostname.
I understood that you want to block the public access to app service (which should be enabled with Azure AD authentication) and by requesting to app gateway the user should be able to reach the app service.
To achieve your requirement, you can follow steps that were mentioned in this blog post and also manage traffic to azure app service.