A minimal example showing a a HttpMessageHandler can be used to intercept and modify HTTP requests done by HttpClient.
FancyService has an dependency on HttpClient which it gets through constructor injection. We initialize the actual instance of HttpClient in Startup.cs in a way that it uses the FancyMessageHandler. All it does is to increment an int on each request done and adds it to the HTTP header.
All FancyService is doing is it's making requests to https://httpbin.org/anything - An API echoing the incoming HTTP request in its response payload. This response is pretty printed in order to show how FancyMessageHandler is able to interact on the payload without FancyService knowing about it.