Code Snippet 2 - https://solusipress.com/menambahkan-combo-box-bertingkat-pada-contact-form-7/

cf7-chained-select.php

<?php 
/*
 * @wordpress-plugin
 * Plugin Name:       CF7 Chained select
 * Plugin URI:        https://solusipress.com
 * Description:       Chained select box for administrative data on Contact Form 7  
 */