Pass URL Parameter to Product Page/Checkout Page

How do I pass a custom parameter to the product checkout page?

Example: I have a custom field named 'serialno' and the product URL is www.MYWEBSITE.com/index.php/option=com_h...ckout=1&product_id=1 - I was hoping that I could simply add '&serialno=12345' to the URL and it would populate the field, but that did not work.

If this can't be done I'd like to be able to populate a custom dropdown field with the results of a MySQL query so I can have the customer select one of their serial numbers and attach the purchase to that item.

One way or the other I need the purchase to be associated with a given serial number of a device that the customer has registered in their profile in the Joomla DB.

Thanks in advance for your help. I really look forward to using this extension to its full potential.

-- url of the page with the problem -- : mywebsite.com/index.php?option=com_hikas...ckout=1&product_id=1
-- HikaShop version -- :Business: 2.3.0
-- Joomla version -- : 3.3.0
-- PHP version -- : 5.x.x
-- Browser(s) name and version -- : ALL/ANY
-- Error-message(debug-mod must be tuned on) -- : No errors