How to send sms from C# windows form application using web api

See more:

C#

Hi, I am working on a c# windows form application, in that I have one form which is used to send the sms to the clients. To send sms I tried the below code but it is showing message failed. I don't know what to do. please correct me, help me.

What I have tried:

if (txtMobile.Text == string.Empty)
           {
               MessageBox.Show("Enter Valid Mobile No");
           }
           else
           {
               using (System.Net.WebClient client = new System.Net.WebClient())
               {

                   try
                   {
                       string url = "http://bulksms.abc.in/API/WebSMS/Http/v1.0a/index.php? " +
                       "&username=" + System.Web.HttpUtility.UrlEncode(txtUserName.Text) +
                       "&password=" + System.Web.HttpUtility.UrlEncode(txtPassword.Text) +
                       "&sender=" + txtSenderID.Text +
                       "&to=" + txtMobile.Text +
                       "&message=" + System.Web.HttpUtility.UrlEncode(txtSmsContent.Text, System.Text.Encoding.GetEncoding("ISO-8859-1"));

                       string result = client.DownloadString(url);
                       if (result.Contains("OK"))
                           MessageBox.Show("Your message has been successfully sent.", "Message", MessageBoxButtons.OK, MessageBoxIcon.Information);
                       else
                           MessageBox.Show("Message Failed", "Message", MessageBoxButtons.OK, MessageBoxIcon.Information);
                   }
                   catch (Exception exr)
                   {
                       MessageBox.Show(exr.Message, "Message", MessageBoxButtons.OK, MessageBoxIcon.Error);
                   }
               }

           }