BuyButton.js Line Items not passed through to checkout

[FrontstageDev]

Hi,

I'm using BuyButton.js to display Shopify products on my website. I need to add a custom line item (text field) to the product which will store a website userid and pass this through to be viewed in the order.

I'm able to display a line item field on the product and the cart but the line item doesn't display at checkout or get passed through to the order.

How have people achieved this?

Thanks!