Code Snippet 4 - https://solusipress.com/menambahkan-combo-box-bertingkat-pada-contact-form-7/

cf7-chained-select.php

<?php
function wpcf7_get_administrative() {
	global $wpdb;
  
  	check_ajax_referer( 'ajax-get-administrative', 'security' );
	
  	if( isset( $_POST['province'] ) ) {
		$province_id = $_POST['province'];
		$sql = $wpdb->prepare( "select * from {$wpdb->prefix}regencies where province_id=%s order by id asc", $province_id );	  
	}
 
  	if( isset( $_POST['regency'] ) ) {
		$regency_id = $_POST['regency'];
		$sql = $wpdb->prepare( "select * from {$wpdb->prefix}districts where regency_id=%s order by id asc", $regency_id );
	}

  	if( isset( $_POST['district'] ) ) {
		$district_id = $_POST['district'];
		$sql = $wpdb->prepare( "select * from {$wpdb->prefix}villages where district_id=%s order by id asc", $district_id );
	}
   
	$rows = $wpdb->get_results( $sql, OBJECT );
	$data = array();
	foreach( $rows as $row ) {
		$data[ $row->id ] = $row->name;
	}
	echo json_encode( $data );

	wp_die();	    
}
add_action( 'wp_ajax_get_administrative', 'wpcf7_get_administrative' );
add_action( 'wp_ajax_nopriv_get_administrative', 'wpcf7_get_administrative' );