convertion of curl to c#

Well, you wouldn't call cURL directly, rather, you'd use one of the following options:

HttpWebRequest/HttpWebResponse
WebClient
HttpClient (available from .NET 4.5 on)
I'd highly recommend using the HttpClient class, as it's engineered to be much better (from a usability standpoint) than the former two.

In your case, you would do this:

using System.Net.Http;

var client = new HttpClient();

// Create the HttpContent for the form to be posted.
var requestContent = new FormUrlEncodedContent(new [] {
new KeyValuePair<string,>("text", "This is a block of text"),
});

// Get the response.
HttpResponseMessage response = await client.PostAsync(
"http://api.repustate.com/v2/demokey/score.json",
requestContent);

// Get the response content.
HttpContent responseContent = response.Content;

// Get the stream of the content.
using (var reader = new StreamReader(await responseContent.ReadAsStreamAsync()))
{
// Write the output.
Console.WriteLine(await reader.ReadToEndAsync());
}