execute : dhcp lease-list
 
dhcp lease-list
Display DHCP leases on a given interface
Syntax
For IPv4:
execute dhcp lease-list [interface_name]
For IPv6:
execute dhcp6 lease-list [interface_name]
If you specify an interface, the command lists only the leases issued on that interface. Otherwise, the list includes all leases issued by DHCP servers on the FortiGate unit.
If there are no DHCP leases in user on the FortiGate unit, an error will be returned.