backend/web/js/fieldWidget.js

$(document).ready(function(){
    $('body').on('click', '.delete-field-item', function(){
        $(this).parent('.form-group').remove();
    });
    $('.add_field').click(function(){
        var block = $('.fields_video_result');
        var sub_block = '<div class="form-group">'+
            '<input type="text"   class="form-control" value="" name="Fields[video]['+ start_i++ +'][value]">'+
            '<span data-id="none" title="удалить" class="glyphicon glyphicon-trash delete-field-item"></span>'+
            '<div>';
        block.append(sub_block);
    });
});