Make dhtmlXComboFromSelect Required

How can I make a dhtmlXComboFromSelect combo required?

I’m not pretty sure what do you mean by “required”, but dhtmlXComboFromSelect return reference to the dhtmlxCombo object, so you can use any events|API calls
dhtmlxCombo has not built in support for “required validation”, but you can use any code in onSubmit method of form, as for normal inputs.