Hi,
There is no option for that with HikaShop Essential. However, with HikaShop Business, you can create custom fields of the table "item" via the menu Display>Custom fields and of the type "text" to allow customers to fill information in them on the product details page, like here:
demo.hikashop.com/index.php/en/hikashop/product-page/with-form
If you want to limit the number of characters, il will be possible by entering a regular expression in the corresponding setting of the custom field and then adding an entry with the namekey check_regex_if_not_required and the value 1 in the table hikashop_config via your phpmyadmin.
If you need to upgrade from your HikaShop Essential to the Business edition, you can do so by purchasing this product:
www.hikashop.com/extensions/upgrade-my-version.html