We need a dropdown with "Add New" option in it. When we click that "Add New" option, a modal dialog should appear to append a new item into the dropdown and selected.
We should be able to customize the event when the "Add New" option is clicked.
Please take a look of the attached video.
i can build you that functionality using Lates tecnologies and the add feature working un real time without affect the performance off the rest of the site