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Can not set attributes in ie
document.getElementsByName('form_name').item(0).setAttribute("onsubmit","add_to_form()");

document.getElementsByName('form_name').item(0).onsubmit = add_to_form;

function listen(event, elem, func) { // cross-browser event listener
  if (elem.addEventListener) elem.addEventListener(event, func, false);
  else if (elem.attachEvent) elem.attachEvent('on' + event, func); // IE
}

listen('load', window, function() {
  var form = document.getElementsByName('form_name')[0];
  listen('submit', form, add_to_form);
});