Settings → Your account ID, at the bottom of the Support section.
It is a long string of letters, numbers and dashes. Tap and hold to copy it.
It is the fastest way for us to find your account, and often the only way if you have not added an email address. Include it whenever you write to us, especially for anything involving lost progress or a payment.
It is not a secret and not a password. Sharing it lets support find your record; it does not let anyone access your account.