fires when the combo list is being populated with data from the server.

combo.enableFilteringMode(true,"dummy"); 
combo.attachEvent("onDynXLS", myComboFilter);
 
function myComboFilter(text){//where 'text' is the text typed by the user in the combo
	combo.clearAll();
	dhtmlxAjax.get("data.php?mask="+text, function(xml){
	    combo.loadXMLString(xml.xmlDoc.responseText);
	})
}

The event is blockable. Returning false or nothing will block requests to the server