Cart - Is there a link to pre-load the cart with a product including line item properties?

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I'm trying to find a way to add a product with line item properties to a user's existent cart using a link (without clearing their existent cart). I learned  I can add items to an existent cart using this query string:[]=variantid 

But does anyone know if there's a way to include line item properties in that link?

My use case would be to include a link in an email to pre-load someone's cart with a product, but this product requires custom line item properties. I know I could include some JS in the cart page and then add whatever I want to the cart after page load, but I'm wondering if there's already a built-in way to do this server-side like in the link above.