RestSharp POST无法正常工作。我正在尝试将用户输入发送到webapi

我正在尝试使用RestSharp客户端发布请求。我的代码如下。目的是将用户输入发送到Web API。我正在VB中工作，并且很难正确格式化请求。尝试执行后，我收到一个错误，阻止了应用程序的启动。对于api，我正在使用Postman。

这是用于调用Web API的代码

private async void StartBtn_Click(object sender, RoutedEventArgs e)
        {
            isWorking = true;
            if (isWorking == true)
            {


                for (int i = 0; i < NumbersListbox.Items.Count; i++)
                {
                    if (ToggleSelected.IsChecked == true && ToggleRandom.IsChecked == false)
                    {

ApiKey = Your_API_KEY                        
var client = new RestClient("https://myurl.com/api/send?key= + Your_API_KEY");                        
browserprimary.Address = client;
client.Timeout = -1;
var request = new RestRequest(Method.POST);
request.AddParameter("phone", "NumbersListbox.Items[i].ToString()");
request.AddParameter("message", "MessagesListBox.Items[i].ToString()");
IRestResponse response = client.Execute(request);
Console.WriteLine(response.Content);;

                    }
                    else if (ToggleSelected.IsChecked == false && ToggleRandom.IsChecked == true)
                    {

                        var random = new Random();
                        int index = random.Next(0, MessagesListBox.Items.Count);

ApiKey = Your_API_KEY;
var client = new RestClient("https://myurl.com/api/send?key= + Your_API_KEY");                        
browserprimary.Address = client;
client.Timeout = -1;
var request = new RestRequest(Method.POST);
request.AddParameter("phone", "NumbersListbox.Items[i].ToString()");
request.AddParameter("message", "MessagesListBox.Items[i].ToString()");
IRestResponse response = client.Execute(request);
Console.WriteLine(response.Content);");                        

                    }


                    await Task.Delay(12000);

                    browserprimary.ExecuteScriptAsync("document.querySelectorAll(\"[data-icon ^= 'send']\")[0].parentElement.click();");


                    if (Delaybox.Text != "")
                    {
                        await Task.Delay(TimeSpan.FromSeconds(Convert.ToDouble(Delaybox.Text)));
                    }
                }
            }

            StartBtn.IsEnabled = false;
            StopBtn.IsEnabled = true;

        }