Not sure how to do the spoiler tag pal or I'd have a go
Look at the icons above the chat box. There are 3 together, the left one looks like a picture, the middle one like film and the one on the right is like a list. Hover your mouse over the one on the right and it will say 'insert' click on this and it will bring up a list of options. Click on the one that says spoiler and it will bring up a box and asked you to type the name of the spoiler. Name it anything you want and click 'continue' then type your message in-between the spoiler tags.