Code Snippet 3 - https://solusipress.com/menambahkan-combo-box-bertingkat-pada-contact-form-7/

cf7-chained-select.php

<?php
function filter_wpcf7_load_provinces( $scanned_tag, $this_exec ) { 
	if( $scanned_tag['name'] == 'province') {
		global $wpdb;
	  	$provinces = $wpdb->get_results( "select * from {$wpdb->prefix}provinces order by id asc", OBJECT );
	  	foreach( $provinces as $province ) {
			$scanned_tag['raw_values'][] = $province->name;  
			$scanned_tag['values'][] = $province->id;  
			$scanned_tag['labels'][] = $province->name;  
		}
	}
	return $scanned_tag; 
}; 
add_filter( 'wpcf7_form_tag', 'filter_wpcf7_load_provinces', 10, 2 );