XML=
<SMS>
<authentication>
<username>account_username</username>
<password>account_password</password>
</authentication>
<message>
<sender>Infobip</sender>
<text>Hello</text>
</message>
<recipients>
<gsm>11111111111</gsm>
</recipients>
</SMS>

XmlDocument doc = new XmlDocument();
XmlNode docNode = doc.CreateXmlDeclaration("1.0", "UTF-8", null);
doc.AppendChild(docNode);

XmlNode MainNode = doc.CreateElement("SMS");
doc.AppendChild(MainNode);

XmlNode authenticationNode = doc.CreateElement("authentication");
XmlNode loginNode = doc.CreateElement("username");
loginNode.AppendChild(doc.CreateTextNode("account_username"));
authenticationNode.AppendChild(loginNode);
XmlNode passwordNode = doc.CreateElement("password");
passwordNode.AppendChild(doc.CreateTextNode("account_password"));
authenticationNode.AppendChild(passwordNode);
MainNode.AppendChild(authenticationNode);

XmlNode messageNode = doc.CreateElement("message");
XmlNode senderNode = doc.CreateElement("sender");
senderNode.AppendChild(doc.CreateTextNode("Infobip");
messageNode.AppendChild(senderNode);
XmlNode textNode = doc.CreateElement("text");
textNode.AppendChild(doc.CreateTextNode("Hello"));
messageNode.AppendChild(textNode);
MainNode.AppendChild(messageNode);

XmlNode recipientsNode = doc.CreateElement("recipients");
XmlNode gsmNode = doc.CreateElement("gsm");
gsmNode.AppendChild(doc.CreateTextNode("11111111111"));
recipientsNode.AppendChild(gsmNode);
MainNode.AppendChild(recipientsNode);


Uri UriObj = new Uri("http://api.infobip.com/api/v3/sendsms/xml");
var request = (HttpWebRequest)WebRequest.Create(UriObj);
request.Host = "api.infobip.com";
request.Method = "POST";
request.ContentType = "application/xml";
var writer = new StreamWriter(request.GetRequestStream());
writer.Write(doc);
writer.Close();

var response = (HttpWebResponse)request.GetResponse();
var streamResponse = response.GetResponseStream();
var streamRead = new StreamReader(streamResponse);

 _result = streamRead.ReadToEnd().Trim();
streamRead.Close();
streamResponse.Close();
response.Close();
return _result;

<?xml version="1.0" encoding="utf-8" ?>