Restrict access to a specific Azure Front Door instance.
Traffic from Azure Front Door to your application originates from a well-known set of IP ranges defined in the AzureFrontDoor.Backend service tag. Using a service tag restriction rule, you can restrict traffic to only originate from Azure Front Door. To ensure traffic only originates from your specific instance, you will need to further filter the incoming requests based on the unique http header that Azure Front Door sends.

Reference:
https://docs.microsoft.com/en-us/azure/app-service/app-service-ip-restrictions#managing-access- restriction-rules