You upload it to itch.io like a regular game and then you register it on the game jam page (on itch.io), as far as I remember. 

Anyway, go to the jam page and click "join" if you haven't already. When the jam starts, there will be a thing on that page to upload / register your game, so you'll figure it out. You can practice uploading projects to itch.io if you never did it before and have concerns about that. (You can keep them private, there's an option for that).