When you created an account using the method Sign in with Apple with your subscription, and you want to log in via the Web version:
You will need to use the same email as you used for Apple Sign In. It can be either the primary email address or the generated email address by Apple if you choose to hide your primary address during login.
Then request an email password reset and finish the password reset process:
Fill in your Board account's email address and click Reset. An email with instructions will be sent to you, then log in via the email.
Note: The email address to use is the one on the Board profile screen.