Additional field at checkout when using a discount code

Hi,

There is a special discount code that gives free shipping, but it is only for people working in a specific building. Those customers should fill in their address as usual, but also add a mailbox number for their office mailbox so that I can deliver them personally. Is there any chance to add a field that only appears after a specific discount code has been entered? I don't want everybody to see that text field.

Thanks!

Custom fields are possible on product pages and (through programming) on basket pages but you have no control over the checkout logic so it would not be possible to selectively hide/show the field in the checkout. 

Okay, thanks. Is there a workaround to this? Some other way of distinguishing these users from others?