Fun fact: If you want to post proof of a prediction without affecting the outcome by revealing it, you can post a unique hash of the text. Then when whatever you predicted happens (or doesn't) you just post the text and anyone can check that it matches the previous hash.
E.g. you could post
"The Suez canal: 12fac09433f6a373019fa2df3a2ad832"
This is the MD5 hash of "The ship will get stuck again"
There are plenty of tools around to calculate the hash of a text, e.g.
or in DuckDuckGo you can just search for the algorithm and your text and it gets calculated for you: