By the way, you grumpy vague bastard, this is fixed.
To add a spoiler, type what you want spoiled, then highlight it, and up in the menu there is an "insert..." option. Click on that and choose "Spoiler". It will ask if you want to give your spoiler a title, like this.
This is the spoiler, bitch!!!
Note that quote, code, and strike-through options are also under the insert menu.