No email is required and you also do not have to login - simply click the link in the first line of the review request email and click the ACCEPT/DECLINE button in the review form indicating that you accept or decline the review.
Before you decline, consider that you would receive Karma Credits for your reviews which are worth up to $100.
If you decline, optionally you can provide the names of other reviewers, refine your reviewing interests, or entering your ORCID, but all this is optional, and your decline request will be registered even if you don't enter those.
See also: