string customer = new JavaScriptSerializer().Serialize(new 
    { 
        mobilenumber = "0870000000",
        firstname = "Hello" 
    }
);
string group = "Hello World";

using (WebClient client = new WebClient())
{
    client.Headers.Add("Authorization", "YOUR_ACCESS_KEY");
    byte[] response = client.UploadValues("https://rest.sendmode.com/v2/import", new NameValueCollection()
    {
        { "importdata", customer},
        { "group", group},
    });

    string result = System.Text.Encoding.UTF8.GetString(response);
}