Getting Response error in The SMS GateWay

string Key = "8btcgcb7a23ubcl";

string secret = "kjojraqp6i1f6t8";

string to = "+919677745308";

string messages = "Hi Vignesh";

string URL = "https://www.thetexting.com/rest/sms/json/message/send?api_key="+Key+"&api_secret="+secret+"&to="+to+"&text="+messages;