In the Ports tab of the Network Interface/Card that contains the port to which a Virtual sub-port will be assigned, highlight the relevant port, and click to create a new virtual sub-port.

In the resulting pool window, select the relevant card to assign the virtual port and click ok, or click cancel to assign to the above card as a default action.

 

The virtual port is visible in the port tab of the Network Interface/Card details view of the assigned card.