Converting an existing window to full screen

What would i need to include in the code if i wanted some text to appear (not as a button) so that when clicked the existing window would convert to full screen leaving just the title bar visible and as a second example leaving the browser top bar visible (as if you had pressed full screen from the browser button).
Thank you

You can't change an existing window in that fashion and it's becoming increasingly clear that you shouldn't alter the user's window in any fashion whatsoever. Just trust your users to make any window size changes required on their own.