Secure a page?

I am selling a downloadable product and using clickbank for affiliate tracking.

They say I need to create a “thank you” page that my customers are directed to after the purchase is confirmed. On that page they will be able to download the product. How do I make sure that no one else can find that page? So that it doesn’t show up randomly on a search site. Once their purchase is confirmed clickbank gives the customers the link…that should be the only way to access that page. Is there a way to secure that page so that it can only be accessed through that link?

Thank you in advance for your help!! 🙂