I did register after I bought the ticket, but it doesn't appear to be possible to tie a pre-purchased ticket to this registration. That would make cancellation easy, but I didn't know about this before I bought the ticket.

Using a hard copy and mailing the information to Hamburg seems like a pretty low-tech solution, and not sure if the info would get there in time.

I'm hoping there's another way. But thanks for the info, if all else fails I'll try the snail mail approach.