I think that when the jam starts there will be a button to upload your game

You can already do something.

For example create your itch page of the game, with this you are able to load some test game and check if everything work correctly

before the jam end you must "link" your itch page of the game to the brackeys jam and all is done.

I already create both the itch.io game page and the repo on github with some pipeline to semplify my work