Hi, on https://itch.io/jam/fuck-capitalism-jam-2026 when you click "Join", it registers your intent to participate, no strings attached you can do it right now.
Then the button changes to "submit your project" which allows you to submit any entry from your itch collection (until the timer runs out). Typically people do that step towards the end of the timer, but you can do it at the beginning and keep working on your game if you're afraid of missing the deadline, I think. You won't be able to update the game after the countdown during the judging period, but this ability will come back after.