Click on the "Rich Text Reply" option or just simply Edit your post and you'll see two eyeballs on the tab above the text box. Click on the one with the line across it to put a spoiler box and then just type your spoiler in the box.
Hardware Comparison Threads:
Current Thread
Is Hardware Getting TOO Powerful?
Older Threads:
PlayStation/Xbox/Switch: 2022 Edition
PlayStation/Xbox/Switch Hardware Battle: 2021 Edition!
PlayStation 4/Xbox One/Nintendo Switch: 2019 vs. 2020
PlayStation 4/Xbox One/Nintendo Switch: 2018 vs. 2019
PlayStation 4/Xbox One/Nintendo Switch: 2017 vs. 2018
PlayStation 4: 2015 vs. 2016 vs. 2017