you should be able to link past purchases with your account by entering your email adress you bought it with here https://itch.io/docs/buying/already-bought
after that you can either play the beta on here or if you want a steam key for workshop access and pre releases of beta 2 you can get a key here https://lizzycat03.itch.io/earthroyale-3/download/Qw8QwsBHy7dKtYW8UX9d5jD02Q6_RfytVU_5XMZ8