Read option sku

products.php

<?php

/** @var $id_product */
/* @var DynamicInputField[] $input_fields */
/* @var $dropdown */

/* @var $dropdown_ref */

use DynamicProduct\classes\models\dynamic_fields\DropDownField;
use DynamicProduct\classes\models\DynamicField;
use DynamicProduct\classes\models\DynamicInputField;

$selected_option = $input_fields['dropdown']->selected_options[0] ?? null;
if ($selected_option) {
    /** @var DropDownField $field */
    $field = DynamicField::getFieldByName($id_product, 'dropdown');
    $option = $field->options[$selected_option] ?? null;
    if ($option) {
        $dropdown_ref = $option['sku'];
    }
}