Hello, I have a website in GitHub:

https://darkpijem.github.io/

when I post it in facebook I get a thumbnail (which I can cancel). How can I change the text and picture of that thumbnail?
It says in the link here :
https://www.facebook.com/help/commun...00814000150949
it has to be connected to meta property in HTML, but there is no meta property in the HTML.

Thx
Darkpijem