You can find the QinQ Settings at Interfaces → Assign → QinQ.
VT AIR currently supports 802.1ad QinQs.
QinQs can only be configured on top of:
- Physical Interfaces
VLAN ID can be set on the interface that you select for the QinQ.
QID can be set on the interface that you select for the QinQ.
Assign to new Interface is an option shown when you create a new QinQ and it will automatically assign the Interfaces as described in Assign Interfaces.