ma usi jQuery? se si, hai la funzione ready:
codice:
$(document).ready(function(){
   var menuContent = $.get($('#categories1').attr('href'), function(data){menuContent = data;});
   $('#categories1').click(function(){
      var menu2 = new Menu(this, {
         content: menuContent,
         width: 216,
         maxHeight: 300,
         positionOpts: {offsetY: -1},
         callerOnState: 'btnMenuOn',
         itemHover: 'hover',
         selectCategories: false,
         topLinkText: 'All Categories'
      }).create();
      return false;
   });
});