We'll set customisation field(s) to only display on the product page (for the user to enter text into) if the customer selects a particular product extra. 

The product extra will need to have the same name on all products that require this functionality. For example the extra name would be 'Add a gift message' or 'Personalise this item'.

We'll use a custom field to enable you to select whether or not to activate this system for each product.