The spoiler?
It’s in your reply window, in the gear icon menu. It’s pushed almost off my screen on the right at the moment, but you can still click it. In there is the magic wand icon, and that puts the [spoiler] [/spoiler] tags around whatever you want hidden.
You can also type them in yourself, and that works too. 
[spoiler] hidden text [/spoiler]
hidden text
And you can have a practice play around with any of the extras in The Sandbox