On purchasing your account you will be asked to create a password to link other devices to your account. Occasionally, if you have already purchased an account in the past at the site you are staying on, the portal may remember your previous password.
If you are not able to remember your previous password, there is a password reset link option from the ‘Already Paid’ signing-in portal (image below). This allows guests to reset their password so that devices can then be added to the account.