By default, you can log into the FortiGate-VM through HTTPS or SSH using the username "admin" and the FortiGate-VM's instance ID as the initial password. SSH also requires your AWS key.
The instance ID is relatively secure as it is visible only within the AWS portal or by running the AWS CLI. However, it may be viewable to those who have access to AWS resources but should not have access to the FortiGate-VM within the same organization. It is strongly recommended to change the initial password the first time you log in or activate the license. You can also create other administrative users using more complex character strings than "admin" in a manner difficult to guess, or add two-factor authentication or other methods to secure login.