Block all traffic to my website UNLESS the traffic has come through my URL shortener.

I would like for my website to be inaccessible to anybody or anything trying to view it unless the user has first come through a separate website of my choosing (my URL shortener). I don't really mind how it's done as long as it works so feel free to do whatever you think would work the best! I am however on quite a tight deadline so the sooner you can start, the better. Thanks in advance!