What's this?
Line item properties are used to collect customization information for an item added to the cart. This information can be collected from the buyer on the product page. To learn more about line item properties, see the line_item object Liquid reference.

Set your form field

Separate your options with a comma.
If you use “Required” with a checkbox, then the checkbox will need to be checked for the customer to add the item to the cart.

Uncheck this if you don't want the captured information to be shown in the order summary on the checkout pages.

Line item properties are not yet shown on the checkout pages, but will be some time in August 2015.

Your form field will look like this:

Grab your code
Copy and paste the code below into your product.liquid file between the <form> and </form> tags. The form field will appear wherever you place the code.