customdropdown.php

См. документацию.

<?php
 
namespace Bitrix\Report\VisualConstructor\Fields\Valuable;

class CustomDropDown extends DropDown
{
    public function __construct($key)
    {
        parent::__construct($key);
    }
 

    public function printContent()
    {
        $this->includeFieldComponent('selectwithpopup');
    }
}