SharePoint - Select List

I have used the following JavaScript to set the default item in a "List form Field"   The script is working and the correct item in the combo box is selected  but the event has not fired to reload the list box. below.  If I select an item manually (i,e. with a mouse) it works fine.  What have I missed.

var elm = document.getElementById("idAttachmentsTable");
          if (elm == null || elm.rows.length == 0)
          document.getElementById("idAttachmentsRow").style.display='none';
         
         
         
  _spBodyOnLoadFunctionNames.push("fillDefaultValues");



function fillDefaultValues()
{
var opt=document.getElementById("ctl00_m_g_f12af616_6ab4_4c5c_a59b_146a2e9b17ae_ff61_ctl00_ctl00_SelectGroup");

var opts = opt.options;
var l = opts.length;
opts.selectedIndex = 4;

      
      }