Add Fortigate interface to VLAN on Trunk to Fortiswitch

I have a Fortigate 60e and recently attached a FortiSwitch 124D-POE with FortiLink.

In the GUI, under Network-Interfaces-Hardware Switch, there is a Trunk to the FortiSwitch with all my VLAN's under it. I have been able to associate the FortiSwitch ports to these various VLANs with no problem, but I now have a need to associate one of the FortiGate physical interfaces to a VLAN defined on this trunk and I cannot figure out how to do it.

Is this possible?

Denny