The submission form categories for the item and stamp design contests are still available, but if you're looking to make any last-minute entries you won't be able to get the player ID needed to submit them from the game as the servers are now down for maintenance.
Fortunately, you can still get your player ID from game logs, in Windows' Documents folder:
(Replace the _STEAM depending on the launcher you use to play the game.)
In here there are several files like ActionLog20210609_00.txt. If you open one of these up, you'll see a line for pretty much everything you do. In most of these, there will be an 8-digit number to the left of your character name. That's your player ID.
Hope that helps!