How to show a page if a parameter comes

i need to show a page contents if i get a parameter only other wise is should be redirected to 404.The idea is to provide users a link to a page that will only accessible via that link(lets say http://www.myblog.com/member/?register=yes).If some one tries http://www.myblog.com/member it will show a 404.What are the options i can use?