Use ReturnUrl in the querystring

If the login form's querystring contains a ReturnUrl, it takes
precedence over the landing page which was defined by default for the
login widget.
To experiment: try accessing a secure page, which redirects you to the
login page first. See:
http://www.sitefinity.com/blogs/stefani-tacheva-s-blog/2014/03/11/redirect-unauthorized-users-to-a-custom-page