It's not the cleanest, but it works. 
Simply add

Simply add
and [/SPOILER} (with the correct closing bracket) around text to make it so people have to click the button to see the text. This is makes it nice so that you don't accidentally spoiler a movie plot, game quest/feature, etc.
Example:
If you are using the advanced editor, you can select the text you want to set as a spoiler by click dragging over the text, then click the ! icon on the editor toolbar. Otherwise, you'll need to type of the tag manually.
Example:
This is spoiler text
If you are using the advanced editor, you can select the text you want to set as a spoiler by click dragging over the text, then click the ! icon on the editor toolbar. Otherwise, you'll need to type of the tag manually.