First we will create the file in our extension like:

getQuery(true);
        $query->select('product_id, product_name');
        $query->from('#__hikashop_product');
        $query->where('product_published = 1');
        $query->where('product_type = ' . $db->quote('main'));
        $query->order('product_name ASC');
        $db->setQuery($query);
        if($products = $db->loadObjectList()) {
            foreach ($products as $product) {
                $options[]  = JHtml::_('select.option', $product->product_id, $product->product_name . ' (' . $product->product_id . ')');
            }
        }
        return JHTML::_('select.genericlist',  $options, $this->name, 'class="inputbox"', 'value', 'text', $this->value);
    }
}

In our extension parameters we will add something like: