public partial class Form1 : Form
    {
        private ICredentials credentials = new NetworkCredential();

		private async void Get(string url)
        {
            using (var client = new WebClient())
            {
                client.Credentials = credentials;
                foreach (ListViewItem item in lstQuery.Items)
                {
                    client.QueryString.Add(item.SubItems[0].Text, item.SubItems[1].Text);
                }
                try
                {
                    var result = await client.DownloadStringTaskAsync(new Uri(url));
                    txtOutput.Text = result;
                }
                catch (WebException e)
                {
                    HandleException(e);
                }
            }
        }

        private async void ExecuteMethod(string method, string url)
        {
            var values = new NameValueCollection();
            using (var client = new WebClient())
            {
                client.Credentials = credentials;
                foreach (ListViewItem item in lstQuery.Items)
                {
                    values.Add(item.SubItems[0].Text, item.SubItems[1].Text);
                }
                try
                {
                    var response = await client.UploadValuesTaskAsync(url, method, values);
                    txtOutput.Text = Encoding.ASCII.GetString(response);
                }
                catch (WebException e)
                {
                    HandleException(e);
                }
            }
        }

	private void btnAdd_Click(object sender, EventArgs e)
        {
            var key = txtKey.Text;
            var value = txtValue.Text;
            lstQuery.Items.Add(new ListViewItem(new string[] { key, value }));
        }