WatchGuard - Setting up VLAN Configuration

To configure Interfaces you need to use Policy Manager

Select Network > Configuration

Select the VLAN tab and click "Add"

Define The VLAN Name, ID then select the "Security Zone" the firewall will belong to and define and set a static IP address with the CIDR notation for the subnet and enable DHCP if required

Define the name of the interface and change the "Interface Type" to VLAN.

This will give you the option to enable "Send and receive tagged traffic for selected VLAN's" which then enables the option to select the VLAN's you wish to use.