How to fire SelectedIndexChanged using MVC

public ActionResult Index()
        {
            var ListItms = dl.BindItems();
            List<SelectListItem> items = new List<SelectListItem>();
            foreach (var itms in ListItms)
            {
                SelectListItem s = new SelectListItem();
                s.Text = itms.ItemCode.ToString();
                s.Value = itms.ItemName.ToString();
                items.Add(s);
            }
            ViewBag.Campaign = items;

            return View();

@using (Html.BeginForm("Index", "Testing"))
{
    @Html.DropDownList("ItmName", new SelectList(ViewBag.Campaign, "Value", "Text"))
}

@Html.DropDownList("ItmCode", new SelectList(ViewBag.Campaign, "Text", "Value"))