On the row of icons above the text box, click the one that looks like a + symbol in a square, then click Spoiler. You'll be prompted to enter a title for the spoiler, but you can leave that blank if you like. Once's that's done, it'll make two text boxes that look like this: [ SPOILER ] Write whatever you wish to put behind a spoiler warning in the space between those two spoiler boxes, and you're done!