Filtered Lookup with FetchXML

function preFilterLookup() {

Xrm.Page.getControl("ral_mapcode").addPreSearch(function ()

	{
     addLookupFilter();
   });
}
function addLookupFilter() {
       var fetchXml = "<filter type='and'><condition attribute='ral_occupied' operator='eq' value='" 'No' "' /></filter>";
		Xrm.Page.getControl("ral_mapcode").addCustomFilter(fetchXml);
    //}
}