add_filter( 'rwmb_meta_boxes', function ( $meta_boxes ) {
    $meta_boxes[] = [
        'title'      => 'Video',
        'fields'     => [
            [
                'id'   => 'video',
                'type' => 'url',
                'name' => 'Video URL',
            ],
        ],
        'validation' => [
            'rules' => [
                'video' => [
                    'only_mp4' => true,
                ],
            ],
        ],
        'message'    => [
            'video' => [
                'only_mp4' => 'Only MP4 is allowed.',
            ],
        ],
    ];
    return $meta_boxes;
} );

add_action( 'admin_print_footer_scripts', function () {
    ?>
    <script>
        jQuery.validator.addMethod('only_mp4', function (value, element) {
            return /mp4$/.test(value);
        }, 'Please enter URL with suffix "mp4".');
    </script>
    <?php
}, 99 );