Creating a hidden/static page

Hi there,

I have a blog where I tend to sell a product from soon. This is my site;
https://thepeoplestories.co.uk/

I will be using Pulley App ( https://pulleyapp.com/ ) to instal a ‘buy it now’ type icon button next to some photos. But to use pulley app, I have to have a ‘transaction failed’ type page that pops up if they decide not to make a purchase. I’m guessing I need to make a hidden page that only pops up when requested. Does anyone know the correct terminology for this?

I have seen a few posts about how to make a hidden page, but they are a little outdated. Would someone be able to tell me how to make a hidden, I can it looks quite easy, and I could just add a plugin but I’m a little nervous about using them, I am a complete novice when it comes to this kind of stuff! I just don’t know which buttons to click in the new 3.5.1 version. Any help would be much appreciated! Thank you!