Do you mean… like this?
If you want the above, you can use:
[spoiler]the spoiler comes here[/spoiler]
If you want specifficaly collapsable spoilers (instead of blur spoilers), you could do this:
some spoiler
spoiler content here
[details="some spoiler"]
spoiler content here
[/details]
You can achieve those options either by writting them just as I demonstrated above, or by clicking in the gear icon at the right of the other editor options above the text area in which you write stuff.