I think there is a cleaner way to do it but this should make it work regardless.
Try Firewall/Input Chain/Add
SourceIP: (The IP of the box that will be connecting to the server (RDP client))
DestinationIP: IP of the server
Protocol Matching: TCP, destination port 3389
Push the “confirm” button
Now add another firewall rule
Source IP: 0.0.0.0/0
Destination IP: IP of the server
Protocol Matching: TCP, destination port 3389
Change Action from ACCEPT to DROP
Push the “confirm” button
Make sure you save the changes