Okey dokey - I cannot do screen shots so this could be fun
When you post reply - go to the 8th icon at the top of the page - the 3 dots with an arrow - click that
3rd in the drop down is ‘spoiler’ - click that and you will get a box appear to type in what your post is about as a header - (so people can chose to read it or skip it)
you will then see [/] brackets - you will need to type your post in between the brackets and then post - once you have done it once it is quite easy - see how you get on with a test