Live update custom_html field

register.php

<?php
add_filter( 'rwmb_meta_boxes', function ( $meta_boxes )
{
	$meta_boxes[] = array(
		'title'  => 'Update custom html field demo',
		'fields' => [
			[
				'name'   => 'Group',
				'type'   => 'group',
				'id'     => 'group',
				'clone'  => true,
				'fields' => [
					[
						'name'  => 'Site title',
						'id'    => 'title',
						'type'  => 'text',
						'class' => 'live-update',
					],
					[
						'name'  => 'URL',
						'id'    => 'url',
						'type'  => 'url',
						'class' => 'live-update',
					],
					[
						'name' => 'Preview',
						'type' => 'custom_html',
					],
				],
			],
		],
	);
	return $meta_boxes;
} );
add_action( 'rwmb_enqueue_scripts', function ()
{
	wp_enqueue_script( 'live-update', plugins_url( 'script.js', __FILE__ ), [ 'jquery' ] );
} );

script.js

jQuery( function ( $ )
{
	$( '#wpbody' ).on( 'change', '.live-update input', function ()
	{
		var $group = $( this ).closest( '.rwmb-group-clone' ),
			title = $group.find( '.rwmb-text-wrapper input' ).val(),
			url = $group.find( '.rwmb-url-wrapper input' ).val(),
			$preview = $group.find( '.rwmb-custom_html-wrapper .rwmb-input' );
		$preview.html( '<a href="' + url + '">' + title + '</a>' );
	} );
	$( '.live-update input' ).trigger( 'change' );
} );