Some details

In this doc https://microsoft.github.io/reverse-proxy/articles/http-client-config.html#httpclient is shows a way to configure a WebProxy, but not mention how to just set the HttpClientHandler.UseProxy to true,
I use a custom configuration to set it to use system proxy for fiddler debugging

                .ConfigureHttpClient((c, h) =>
                {
                    
                    h.UseProxy = true;
                })

and I hope I can use the build in configuration for that