Add a line of JavaScript to an URL in the browser

I am trying to force fullscreen mode in the Chrome app. I've found a post that I cannot comment on for lack of reputation . Apparently, I must paste a line of script in the URL of the website, but I cannot seem to get it right whatsoever (I pasted it both before and after the URL). I'd like to have a step-by-step tutorial of how to proceed, with some visuals, if this is still the only alternative available.