It is because he has declared strUrl two times. So, do like below...
string strUrl = "http://api.mVaayoo.com/mvaayooapi/MessageCompose?user=t122kes@gmail.com:9768214005&senderID=TESTSMS&receipientno={0}&msgtxt={1}&state=4";
strUrl = string.Format(strUrl, txtNumber.Text, txtMessage.Text);