It was confusing.
You have to claim it on the Twitch website first and then you have to claim it again on the Runescape Website.
If you look at the instructions it provides a link to the part of the Runescape website to go to. The page is weird and it says something like "claim your reward" but it does not look like something you would normally click on.
Hope this helps. If I didn't make sense I can try to clarify.
11-Jul-2019 08:43:30