To add a note to an order you will need to click and hold on the marked product, and from there a new window will appear, so you can type the note.
*Please note that once the order was saved, and the preparation receipt was already sent to the kitchen, you can no longer add notes to that specific order.
After you added the note to the product, this will have an information icon near the name, suggesting that this product has additional information attached.
Also the note will be printed on the non-fiscal printer.