is it possible to use a dependent dropdown in Lightbox? See: http://demos.krajee.com/widget-details/depdrop
Lightbox API allows you defining custom controls, so you should be able to create a lightbox wrapper for jQuery dependent dropdown.
Here is related docs article: https://docs.dhtmlx.com/scheduler/custom_lightbox_editor.html
Also, there is a working example if dependent dropdowns implemented in plain js/html in scheduler samples package:
https://docs.dhtmlx.com/scheduler/samples/02_customization/26_linked_selects_in_lightbox.html