nostr:npub1nu9m6k6dca28f9humvq0ad334a3czu0qrevw0pd4ml2pzaumx0mqyr3ars has the right of it. If you post the hash of your prediction, that note is going to be signed by your key, proving that you indeed posted it. Then you can post the text used to create the hash once your prediction comes true, which will be signed by the same private key, and you can reference the original post containing just the hash, too.

Bonus points if you also use open timestamps to prove you wrote the note on a certain date, since you can create a note with an arbitrary timestamp on Nostr.