Ubuntu on OCI - password after screen lock?

Summary:

What is the ubuntu password?

Content (required):

Of course I logon to my ubuntu VM on OCI with SSH but for the screen lock function in ubuntu I need to have the password for the ubuntu user. If I lock the screen using the lock function in ubuntu, to unlock I need the password. Which I don't know. The only thing that then can be done is to reboot the VM.

This is very, very cumbersome so I downloaded a gnome extension to remove the lock function but locking the screen is a preferable practice when you leave your desk. This function should not be removed.