Why can't I track my pixel on a 'thank you' page?

Hello,

Once my users subscribe via double opt-in, they get presented a ‘thank you’ page with a dynamic URL like this one https://marcribellenas.activehosted.com/f/confirm.php?id=14&s=499d6eff15354a17e6e544361b1c3a4f

I’ve tried to insert an event in this page in order to exclude these people from FB ads I run, using the Pixel Caffeine plugin. Although it’s worked on my web pages, there’s no event in this one.

Any idea why the event is not showing?