Apparently there's a code you can enter, but nothing to tell you this and no information on where to get the code.
I guess you have to be logged in to the site to see it, but I tried to register an account (guess I didn't already have one) and the confirmation link it sends me has already expired. If I click the "email me a new password" link then it sends me another link that, when clicked, complains about having expired.
YHA must use the same system as the links look very similar and I complained to them about exactly the same problem earlier in the week.