Accessing the client certificate from App Service.
If you are using ASP.NET and configure your app to use client certificate authentication, the certificate will be available through the HttpRequest.ClientCertificate property. For other application stacks, the client cert will be available in your app through a base64 encoded value in the "X-ARR-ClientCert" request header. Your application can create a certificate from this value and then use it for authentication and authorization purposes in your application.
Reference:
https://docs.microsoft.com/en-us/azure/app-service/app-service-web-configure-tls-mutual-auth
