I can make it work using checkout permalinks as detailed here:
https://community.shopify.com/topic/613702
But it doesn’t seem possible to do the same by putting in a form input on my cart page such as:
Other inputs for order attributes pass through to the order just fine:
or
I’ve also tried to do it via the the post URL of my form but this doesn’t work either:
I'd prefer not to go via the API (or build up a permalink URL based on what the user inputs into my attribute fields on my cart page), I want to achieve this using pure HTML/javascript. But from what I can see, it isn't possible. Any ideas?