This can get confusing because there's the square enix account which goes on top of your PlayOnline account. So you have a playonline ID and playonline password. The ID is assigned to you by playonline. It is generally 4 letters and 4 numbers. The playonline password is set by you. Generally, you only enter this once when setting up PlayOnline and you save it so you don't have to use it anymore. What you enter to log in now is your SquareEnix Account password, which you set when you created your square enix account.

There's also the "Member password", which is completely optional, and specific to your local machine, and entered before your SquareEnix Account information if you have this turned on.