When you go to https://signup.wow-europe.com/, click account creation at the top right navigation bar, from there it should give you Create Account and Manage Existing, click Create Account and it should ask for an authentication key for your 10 day trial/full subscription/guest pass key.

edit:gave American server info at first should be updated properly now, also i gave the direct link to account creation instead of the WoW website so just click create the key should be in stage 2.