You have two options.
1) First create the game page and then you add it to the jam...
2) or, you press the join jam, and then proceed to create the game page.
Either way is ok. But take note that the game won't be visible in the comp web page till halloween.
Also, maybe you want to cast an eye to this, to improve accessibility to your game:
https://intfiction.org/t/technical-advice-for-hosting-at-itchio/12787