Configuration, Services
This controls traffic from outside coming into your system. Either configure the workstation as a 'Static NAT' or create a Custom Service entry. If you choose the custom method, the default ports used by Remote Desktop are 3389 both TCP and UDP so set up forwarding entries for these ports to the local IP of the workstation.
Configuration, Firewall
If not disabled, allow TCP and UDP traffic on 3389
Remember to open up 3389 on any software firewall you have running on the client.