Define in the Lightbox two dependents combobox

How I can define in the Lightbox, two nested combobox (dependents combobox)?

there is no built-in option for linking selects in lightbox. However, you can define onchange handler on a primary select and update the secondary select manually
Please check the example of this functionality on dhtmlxScheduler forum (lightbox settings are very similar for both components)
Example: … htbox.html

What should be changed in the following method to work on Gantt?

    scheduler.attachEvent("onBeforeLightbox", function(id){
		var ev = scheduler.getEvent(id);     [b]   ----> (gantt.getEvent(id) Not exist.  What should I write here?)[/b]
		if (!ev.child_id) {
			var parent_id = ev.parent_id||parent_select_options[0].key;
			var new_child_options = child_select_options[parent_id];
		return true;

you need to change the host object (gantt, instead of scheduler), and the related methods
scheduler -> gantt
scheduler.getEvent -> gantt.getTask
scheduler.formSection -> gantt.getLightboxSection

getLightboxSection in gantt does not have a ‘control’ property, so you’ll have to retrieve the select element manually: var select = gantt.getLightboxSection("child").node.getElementsByTagName("select")[0];