HttpClient is available through NuGet: Microsoft HTTP Client Libraries.

string requestUri = "<http://www.example.com>";
string requestBodyString = "Request body string.";
string contentType = "text/plain";
string requestMethod = "POST";

var request = new HttpRequestMessage
{
    RequestUri = requestUri,
    Method = requestMethod,
};

byte[] requestBodyBytes = Encoding.UTF8.GetBytes(requestBodyString);
request.Content = new ByteArrayContent(requestBodyBytes);

request.Content.Headers.ContentType = new MediaTypeHeaderValue(contentType);

HttpResponseMessage result = client.SendAsync(request).Result;
result.EnsureSuccessStatusCode();