Explanation:
You can restrict access to your Azure App Service app by enabling various types of authentication for the app. One way to set up authentication is to request a client certificate when the client request is sent by using Transport Layer Security (TLS) / Secure Sockets Layer (SSL) and to validate the certificate. This mechanism is called mutual authentication or client certificate authentication.
Step 1: Upgrade the pricing tier of the App Service Plan.
Prepare your web app
If you want to create custom TLS/SSL bindings or to enable client certificates for your App Service app, your App Service plan must be in the Basic, Standard, Premium, or Isolated tiers.
Make sure that your web app is in a supported pricing tier.
Changing from a D1 (Shared) tier to a Basic tier on Azure App Service is an upgrade. While both tiers are for web apps, the Basic tier provides dedicated resources and more features compared to the shared environment of the D1 tier.
Step 2: Modify the client certificate mode of the Configuration settings of App1.
Enable client certificates
When you enable client certificates for your app, you should select your choice of client certificate mode. The mode defines how your app handles incoming client certificates.
Step 3: Modify the certificate exclusion path of the Configuration settings of App1.
Exclude paths from requiring authentication
When you enable mutual authentication for your application, all paths under the root of your app require a client certificate for access. To remove this requirement for certain paths, define exclusion paths as part of your application configuration.
Reference:
https://learn.microsoft.com/en-us/azure/app-service/app-service-web-configure-tls-mutual-auth