Custom domain suffix capability is now available in App Service Environment (ASE), an Azure App Service feature that provides a fully isolated and dedicated environment for running App Service apps securely at high scale. Your apps are not restricted by the DNS settings for your App Service Environment’s default domain suffix to only be accessible by those domain names. Custom domain suffix is an internal load balancer (ILB) App Service Environment feature that allows you to use your own domain suffix to access the apps in your App Service Environment.
For more information about the capabilities available, visit the Custom domain suffix for App Service Environments documentation webpage.