The first time you went to that server it must have asked you for an account name and for a password (twice). That's where your password was defined.
If you now go there again and create another account you must re-enter the password.
Alternatively, create on the "Settings" cogwheel at the top, then on "Wallet management console" at the bottom, then on "New Wallet". This will create a new wallet with a new password. Use that password to create accounts in the new wallet.
(You have have several wallets in a browser. Each wallet is protected by its own password and can contain several accounts.)