The tool bar at the top of the text box. If you click the last set of three little dots it’ll pull up some stuff. Then click the eyeball with a line through it, and it will pull up another box. This is where you title what you want your spoiler to be. then it will put this SPOILER="."] ! [/SPOILER]” in your text box and you put whatever text/image in between the two brackets where I have the exclamation point.
77