2 ways to input the spoiler code:
[ spoiler ] insert whatever [ /spoiler ]
^ note: remove the space between the brackets in order to see this effect.
2. See the picture-like icon right beside the Font box?
Use that. Then click on the drop-down box for some choices. There should be a Spoiler choice.
Example of how it should look like:
Just a friendly reminder guys (ﾟωﾟ)