By default, root is the management VDOM. You use the management VDOM to access the global settings for the FortiGate as well as the settings for each VDOM.
To configure an interface to connect to the management VDOM, go to Global > Network > Interfaces and edit an interface (in the example, mgmt).
Enable Dedicated Management Port and add the management computers as Trusted Host.
Set Administrative Access to HTTPS, PING, and SSH.