This doc explains how to add extra Shopify line item properties to a customers cart and resulting Shopify order.

For both cart and checkout redirects (when using a storefront token), PreProduct can add additional line item properties to an order’s line items.

This can be done by setting a ppCustomLineItemProperties variable on the window object. It has to be a Javascript object.

window.ppCustomLineItemProperties = {
  preorders_are: `great`
}