Took me a while to figure out as well. When you're posting a comment, click the "warning" symbol in the bottom right. It will put this in the text box. The second instance of "spoiler" (underlined in red) is the text that shows before clicking, you can change that to anything you want. The line underneath it (underlined in blue) can be deleted and the spoiler content goes there.

So this setup comes out like this: