How do I make GetCaseButton work when clicked?

[User 11369001]

I have a new web app that is based on an old desktop app. I need help on how to make changes to old GetCaseButton_Click desktop app code and make it work for new web app. The old code is using Combobox and MessageBox for error message. In my web app aspx code I am using DropDownList and ClientScript for error message. Is there a better way of displaying error message in web app to replace MessageBox? How do I change Combobox code to DropDownList? The code is very long 200 lines. So I have broken it down to several questions. The GetCaseButton_Click that I need to make changes to so I can use DropDownList instead of Combobox.

private async void GetCasesButton_Click(object sender, EventArgs e)
{
#region Required Field Validation
if (CaseNumbersTextBox.Text.Length < 1)
{
string myStringVariable = "Case number textbox cannot be empty.";
ClientScript.RegisterStartupScript(this.GetType(), "myalert", "alert('" + myStringVariable + "');", true);
}

        ComboboxItem requestorItem = new ComboboxItem();
        requestorItem = (ComboboxItem)RequestorComboBox.SelectedItem;



        ComboboxItem reasonItem = new ComboboxItem();
        reasonItem = (ComboboxItem)ReasonComboBox.SelectedItem;

        if (requestorItem.Value < 1)
        {
            MessageBox.Show("Please select a Requestor.", "Search Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            return;
        }

        if (reasonItem.Value < 1)
        {
            MessageBox.Show("Please select a Reason.", "Search Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
           
           
            return;
        }
        #endregion

        #region Parse case number entries
        string userEnteredCaseNumbers = CaseNumbersTextBox.Text;

        userEnteredCaseNumbers = userEnteredCaseNumbers.Replace("\\r", ",");
        userEnteredCaseNumbers = userEnteredCaseNumbers.Replace("\\n", ",");
        while (userEnteredCaseNumbers.Contains(",,"))
            userEnteredCaseNumbers = userEnteredCaseNumbers.Replace(",,", ",");

        List userEnteredCaseNumberList = new List();
        userEnteredCaseNumberList = userEnteredCaseNumbers.Split(',').Where(x => x.Length > 0).ToList();
        userEnteredCaseNumberList = userEnteredCaseNumberList.Select(s => s.