Join KeyValuePairs into an x-www-formurlencoded string, then POST it to a URL.

gistfile1.cs

public async Task<string> YourMethod()
{
    //Create a list of your parameters
    var postParams = new List<KeyValuePair<string, object>>(){
                new KeyValuePair<string, object>("FirstParameter", "First Value") ,
                new KeyValuePair<string, object>("SecondParameter", "Second Value")
            };

    //Join KVPs into a x-www-formurlencoded string
    var formString = string.Join("&", postParams.Select(x => string.Format("{0}={1}", x.Key, x.Value)));
    //output: FirstParamter=FirstValue&SecondParamter=SecondValue

    //Encode form string to bytes
    var bytes = Encoding.UTF8.GetBytes(formString);

    //Create a POST webrequest
    var request = WebRequest.CreateHttp("https://www.example.com");
    request.Method = "POST";
    request.ContentType = "application/x-www-form-urlencoded";

    //Open request stream
    using (var reqStream = await request.GetRequestStreamAsync())
    {
        //Write bytes to the request stream
        await reqStream.WriteAsync(bytes, 0, bytes.Length);
    }

    //Try to get response
    WebResponse response = null;
    try
    {
        response = await request.GetResponseAsync();
    }
    catch (WebException e)
    {
        //Something went wrong with our request. Return the exception message.
        return e.Message;
    }

    //Get response stream
    using (var respStream = response.GetResponseStream())
    {
        //Create a streamreader to read the website's response, then return it as a string
        using (var reader = new StreamReader(respStream))
        {
            return await reader.ReadToEndAsync();
        }
    }
}