Hi. My name is Viktor. We've just read your task and our Wordpress developers can develop a multi-select checkbox on your Wordpress form.
We have good experience with CMS Wordpress and its plugins (Gravity Forms, Contact Form7, Woocommerce, Custom Fields, etc), core Php, Javascript, Jquery, Bootstrap, HTML, CSS and etc.
We'd be glad to discuss your project via chat to clarify all requirements. Thank you.