In this article, the author demonstrates how to create a custom ITwilioRestClient using Dependency Injection (DI) in ASP.NET Core 2.1, which allows for customization of HTTP requests to the Twilio REST API. The custom client is created by injecting an HttpClient into its constructor and adding a custom header to all outgoing requests. The custom client is then registered with the DI container, allowing it to be injected into any class that requires an ITwilioRestClient. The author also shows how to test the API using Postman by sending a JSON request to the controller. This technique can be used for various purposes, such as testing classes, extending the CustomTwilioClient further, or injecting it into other services.