How do I display a default message in dropdownlist?

<asp:DropDownList ID="RequestorDropDownList" runat="server" Font-Size="8.25pt" Height="20px" ToolTip="Requestor" Width="160px" SelectMethod="GetRequestorEmail"
DataTextField="DisplayName" DataValueField="Email"></asp:DropDownList>

public async Task<IEnumerable<GetRequestorInfoModel>> GetRequestorEmail()
{
   try
   {
      var requestors = await FTACaseReseting.Controllers.RequestorInfoController.GetAllRequestorInfoes();
      return requestors.OrderBy(x => x.DisplayName);
    }
     catch (Exception ex)
     {
      string errorMsg = string.Format("An error has occured in {0}. \nException:\n{1}", "PopulateRequestorComboBox()", ex.Message);
      Response.Write("<script>alert(" + HttpUtility.JavaScriptStringEncode(errorMsg, true) + ")</script>");
      return Enumerable.Empty<GetRequestorInfoModel>();
     }
}