show feedback Modal when user/guest votes down

Yes, you have to attach a JavaScript event to the vote down button.

Can’t tell you the exact code since it depends on your modal.

I would start with something like this:

jQuery(document).ready(function() {
    jQuery(".thumbs-rating-container .thumbs-rating-down").on("click", function(event){
      // Do something
    });
});

Awesome thanks Ricard, I will try to figure it out and get back to you if I need more tips.
Cheers,
Piet