select control rendered using htmltextwriter not available in Request.Form in Asp.net

Question

I needed a listbox with groups and for that I placed a literal control in page and I rendered a select control using HTMLTextWriter to the literal control. It rendered perfectly.

Problem is I am not able to get the select control from Request.Form object. The id of select control is missing in Request.Form.

Why is it so and how can I get the user selections in code behind? Code given below.

Dictionary<string,List<string>> locations =  GetLocations();

      StringWriter stringWriter = new StringWriter();
      HtmlTextWriter html = new HtmlTextWriter(stringWriter);

      html.AddAttribute(HtmlTextWriterAttribute.Multiple, "multiple");
      html.AddAttribute(HtmlTextWriterAttribute.Id, "ddlLocations");
      html.RenderBeginTag(HtmlTextWriterTag.Select);
      foreach (string region in locations.Keys)
      {
          html.AddAttribute("label", region);
          html.RenderBeginTag("optgroup");
          //add countries
          foreach (string country in locations[region])
          {
              html.AddAttribute(HtmlTextWriterAttribute.Value, country);
              html.RenderBeginTag(HtmlTextWriterTag.Option);
              html.Write(country);

              html.RenderEndTag();//close option
          }
          html.RenderEndTag();//close optgroup

      }
      html.RenderEndTag();//close select

      literalForLocationDdl.Text = stringWriter.ToString();