To connect the form, you need to use your letter processing file, for example send.php. Then, in the scripts.js file, you should add the processing of this form via js. For example, the code responsible for the “send” button in the modal window:

$("#modal_form .js-submit").on("click", function(e) {
        var thisBtn = $(this);
        
        $(".success .text").text("CLOSE");
       $("#modal_form .form_wrap").hide();
        $("#modal_form .success").show();
        $("#modal_form .title").text($("#modal_form .title").attr("data-success"));
    });

You need to supplement this script with your form processing code. Now the form is not connected and has a demo character