You will access the standby firewall for the first time, so you should also register the appliance before proceeding with the rest of the configuration.
Navigate to Network > System > Interfaces, and create the HA interface with the custom zone "HA" using the below IP address schema details (SonicWall uses X2 Interface in the customer template).
Security Type: Trusted
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The IP Address is the ha-HA2-Interface-X2 found on the Networking tab.