public class MyClass{
            public NewResponse MySepcificHeaderApi(HttpRequestMessage request) {
               if( OnSomeCondition()){
                   // i want to test whether this header added or not
                   request.Headers.TryAddWithoutValidation("X-Specific-Header", "some values");
               }    
               var httpClient = httpClientFactory.CreateHttpClient("Some name");
               var response = httpClient.SendAsync(request).GetAwaiter().GetResult();
               
               // here we lost X-Specific-Header, as we taken only status code 
               // from the original response.
               return NewResponse(response.StatusCode); 
            }
        }

// arrange
var request = new HttpRequestMessage(Get, "some uri");
// act
var response = new MyClass().MySepcificHeaderApi(request);
//assert
response.Headers.should().Contain("X-Spcific-Header"); // will fail, because the actual requet details are missed out from original response in MyClass.MySepcificHeaderApi()

// arrange
CustomWebApplicationFactory<Program>? webApplicationFactory = new CustomWebApplicationFactory<Program>();
HttpClientInterceptorOptions? interceptorOptions = webApplicationFactory.InterceptorOptions;
interceptorOptions.BeginScope();
var httpClient = webApplicationFactory.CreateClient();
var headers = new Dictionary<string, string>();
new HttpRequestInterceptionBuilder()
	.Requests()
	.WithInterceptionCallback(i =>
	{
		foreach (var httpRequestHeader in i.Headers)
		{
			headers.Add(httpRequestHeader.Key,
				httpRequestHeader.Value.FirstOrDefault());
		}
	})
	.ForPost()
	.ForHttp()
	.ForHost("localhost")
	.ForPath($"some uri")
	.ForRequestHeader("X-Specific-Header", "some values")
	.Responds()
	.WithStatus(HttpStatusCode.OK)
	.RegisterWith(interceptorOptions);
// the above HttpRequestInterceptionBuilder will return OK status, only is the request contains the "X-Specific-Header, that way we are asserting the header is added.
var request = new HttpRequestMessage(Get, "http://localhost/some uri");
// act
var response = new MyClass().MySepcificHeaderApi(request);
//assert
reponse.StatusCode.Should().Be(Ok);
headers.Should().ContainKey("X-Specific-Header");
headers["X-Specific-Header"].Should().Be("some values");