Step 1: Under the Network tab click on Servers underneath Inbound Access
Now click on Add Server and give it a name and enter the LAN IP address of the device you want to forward traffic to and save it
Step 2: Click on Services underneath Inbound Access and then click on Add Service. Here you will give it a name and select Port Mapping and choose which WAN connections/IP addresses to listen on
Then just select the Server LAN IP you just created in the previous step to forward the traffic to
Step 3: Save, apply changes, and you are off to the races 