@Anonymous User
Welcome to the forum. With some of these offers you can fix them yourself by looking for offer codes in the URL and entering these manually, but because this particular offer is a joint promotion between NOW TV and Sky Box Office Online I'm not sure exactly how they track it all, and I've not received the offer myself to look at.
To be on the safe side if clicking the offer button keeps bringing up the blank screen I'd recommend popping onto live chat, explaining the offer you've got and asking if they can help you get it.
You can get to live chat from the Contact Us page
https://help.nowtv.com/get-in-touch
(expand the Anything Else section at the bottom for options)
