var customer = new ExpandoObject(); // Or any dynamic type
data.username = "test";
data.password = "test";
data.action = "shorturl";
data.format = "json";
data.url = "www.google.com";
var http = new HttpClient();
http.Post("http://yourlstest789.byethost7.com/yourls-api.php", data, HttpContentTypes.ApplicationXWwwFormUrlEncoded );