If you are manually trying to type the spoiler tags out... you aren't doing it right.
Manually: Just insert the equal sign in the first bracket. then if you want a text header for your spoiler, just type in a few words after the equal sign.
E.g:
Automatic: Just go to the advanced mode and click on the spoiler wrap icon in the editing toolbar. Easy peasy from there.
Just a heads up, you might want to prepare to update this soon. It suffers from no-progress bug that is currently under rapid debugging.
Scheduled to be fixed on May 1st at 22:00 JST. 聖騎士リアナ