Solo? Leave blank. To host: tap New code and read it out. To join a friend: type their exact code (don't tap New code). Both screens must show the same code.
You can still swap mid-game with keys 1–4 / Q.
Menu
Keyboard: Move ←→↑↓ / WASD · Fire Space
· Magic Shift/M · Hero 1–4 / Q.
Phone: on-screen stick (left) + FIRE / MAGIC / SWAP.