How do I create a route between DMZ and LAN

I have a VM that sits in it's own DMZ (192.168.57.0/24) - it's default GW is the DMZ interface of the firewall of 192.168.57.1

I then have my clients and other VM's sat on my LAN (192.168.16.0-192.168.18.255/22) - default GW is LAN interface on firewall of 192.168.16.1

I want to create a route which allow me to RDP onto the DMZ server from my client PC on the LAN. 

How do I go about doing this?

I am using a Fortigate 110c physical appliance.