Hi,
You can configure a custom item field via the menu Display->Custom fields in order to display a text box on the product page above the "add to cart" button which the user will have to fill in order to add the product in the cart.
It's also possible to create custom order fields in the same way which would be displayed during the checkout instead.
The difference between the two is that with the first option, you will be able to have one text per product in the cart where, with the second option, you will have one text for all the products in the cart.
Both possibilities are only available in the Business Edition of HikaShop.