Administrative access
This is rather an ordinary consideration than AWS-specific to secure the FortiGate-VM and protect it by configuring allowed and restricted protocols and ports in corporate security scenes.
One example is to configure the local admin access to one of the FortiGate-VM's local network interfaces. Log into the GUI, go to Network > Interfaces, then chose the desired port to configure under Administrative Access.
To configure general firewall policies to protect VMs in the networks, refer to Setting up a Windows Server in the protected network or the FortiOS documentation for details.