Protecting an entire page

I want to create a page that is only accessible using a link. I'm setting up an optin page and I want it only accessed when people use the link from my ad campaign. I installed Password Protect but I don't see how to protect the entire page. I have Membership, but I don't want to require a level of membership. Unless I can do it by requiring no membership? But I don't necessarily want to exclude members from the page. Any thoughts on how I can accomplish this? I'm open to any suggestions.