"Coupon" is also used in this sense in advertising and marketing. See these definitions:

coupon ad = advertisement with a form attached, which is to be cut out and returned to the advertiser with your name and address if you want further information about the product advertised (annons med svarskupong/svarstalong)
reply coupon = form attached to a coupon ad, which must be filled in and returned to the advertiser (svarskupong) (see page 67)
http://books.google.co.uk/books?id=1Yb2eAR3ROQC&pg=PA8&lpg=P...

The Swedish terms in brackets are those given in the English-Swedish version of this dictionary by the same author, the paper version of which I have seen.

COUPON: Part of an advertising promotion intended to be completed by the customer and returned to the advertiser.
http://www.greatmailinglists.com/direct-mail-glossary/

Coupon — Reply form within the design of a printed ad. May also refer to a reply card in a direct mail package.
http://www.directcreative.com/direct-marketing-glossary.html

I would avoid the use of marking as that doesn't really mean very much to the English-speaking reader in this context. Märkning is always a bit tricky. I think it is referring here to providing personal details or information and so something like "to be completed by" or "to be filled in by" should be sufficient. It would be interesting to see what others think.