Looks like the system adds a number of non-printing unicode characters into the string. In this thread title, for example, the textmark is stored in the phrase "message has a hidden message inside." (Thanks, textdiff!)
Specifically, it adds non-printing unicode characters repeatedly. I'd guess these form a unique series, which is mapped to a record in the textmark database.
If I wanted to automatically remove the textmark from a text, in order to steal your content, I would write a script to automatically remove any non-printing unicode characters that appear in it. This StackOverflow answer includes a script achieving precisely that task: https://stackoverflow.com/a/11598864/6226586
If you wanna look it's on GitHub. Here are the links.
Is the message really hidden in this message? Or are you just doing the lookup your database of hidden messages based on the hash of the public message?
Regarding the website UX - after I pasted the message to your text field I thought the hidden message is the youtube video. But that didn't make sense to me and only later I understood the message is above the video.
If you are interested in more UX feedback, feel free to visit https://usability.testing.exchange which is a free community platform built exactly for this purpose.
Also, looking at the payment options - I'm missing a pay-per-message approach. For your consideration.
Also, all those trademark symbols makes this look comically unprofessional.