I'm looking to find out whether I can pre-populate a custom field on the checkout page. I can stick custom fields on the page, and of course fill them, and get them to appear in the custom fields in the transaction. But I'd like to pre-fill them.
The premise behind this is as follows:
1. Cart page sits on our site and loads the cart in using JSONP (straight out of the example in the docs)
2. The checkout button on here fires the user straight into the checkout page (e.g. missing the /cart page).
3. This checkout button can (as per the docs) pre-populate most of the fields (via POST params or GET using the querystring).
The idea is that I'd like to pass an reference into the order.
Any help on this would be awesome. Thanks.