Adding Http Headers to HttpClient

I need to add http headers to the HttpClient before I send a request to a web service. How do I do that for an individual request (as opposed to on the HttpClient to all future requests)? I'm not sure if this is even possible.

var client = new HttpClient();
var task =
	client.GetAsync("http://www.someURI.com")
	.ContinueWith((taskwithmsg) =>
	{
		var response = taskwithmsg.Result;

		var jsonTask = response.Content.ReadAsAsync<JsonObject>();
		jsonTask.Wait();
		var jsonObject = jsonTask.Result;
	});
task.Wait();

Create a HttpRequestMessage, set the Method to GET, set your headers and then use SendAsync instead of GetAsync.

var client = new HttpClient();
var request = new HttpRequestMessage() {
    RequestUri = new Uri("http://www.someURI.com"),
	Method = HttpMethod.Get,
};
request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("text/plain"));
var task = client.SendAsync(request)
	.ContinueWith((taskwithmsg) =>
	{
		var response = taskwithmsg.Result;

		var jsonTask = response.Content.ReadAsAsync<JsonObject>();
		jsonTask.Wait();
		var jsonObject = jsonTask.Result;
	});
task.Wait();

When it can be the same header for all requests or you dispose the client after each request you can use the DefaultRequestHeaders.Add option:

client.DefaultRequestHeaders.Add("apikey","xxxxxxxxx");      

To set custom headers ON A REQUEST, build a request with the custom header before passing it to httpclient to send to http server. eg:

HttpClient client = HttpClients.custom().build();
HttpUriRequest request = RequestBuilder.get()
  .setUri(someURL)
  .setHeader(HttpHeaders.CONTENT_TYPE, "application/json")
  .build();
client.execute(request);

Default header is SET ON HTTPCLIENT to send on every request to the server.