You always get the access data to your databases after creating one in the customer interface (PLESK) under the menu item Databases. No databases are preset.
The password is displayed in the customer interface when the database is created and can be changed there at any time. The database name and the user name are freely selectable. Please note that you share the namespace of the databases with other customers. Therefore it can happen that a certain database name is already assigned to another customer.
Important: We can't answer inquiries about a lost password, because this data is stored encrypted. You must therefore assign a new password for the database. A little tip: Often the password can still be found somewhere in your scripts :)