To use user scripts you need to first install a user script manager. Which user script manager you can use depends on which browser you use PC & Mobile https://tampermonkey.net/
Press "Dashboard" and then in the top there should be a tiny + sign (left from "installed user scripts")
Erase everything and copy ALL this code this code
and paste it into the addon.
Now log in into the official telegram web client
and you should see a download icon.