Select Control ! How to add new option?

Gantt
#1

example https://select2.org/tagging

#2

Hello Weerayut,
There is no built-in way to do that.
You need to create a custom element in the lightbox and implement a solution that will use a third-party library:
https://docs.dhtmlx.com/gantt/desktop__custom_editor.html

Here is an example with the select2 library:
http://snippet.dhtmlx.com/248e7a34f

And here is a similar example with the Amsify plugin:
https://snippet.dhtmlx.com/1867d0f9b