First click/tap on the 3 dots shown below. (I use a phone, so my formatting of the toolbar might be slightly different from yours)
Then press this button shown:
It'll ask for a title. (You don't have to put a title. If you don't want a title, press "Continue" to skip the title prompt).
After putting the title, type what you want to be inside the spoiler where the green line is below.
Then, when your post is uploaded it will show a spoiler button that says
Spoiler: *your title goes here*. (If you didn’t put a title, it will just say
Spoiler). When you press the button, it will show what you typed for the spoiler earlier.
There is also another type of spoiler you can create that is a little different. It is shown below: