In order to finish creating a VxRail VI workload domain, you must add the primary VxRail cluster to the workload domain.

There are two ways to add the primary VxRail cluster to a workload domain, depending on your use case.
Use Case Method
You have a single system vSphere Distributed Switch (vDS) used for both system and overlay traffic. SDDC Manager UI
You have two system vSphere Distributed Switches. One is used for system traffic and one is used for overlay traffic. MultiDvsAutomator script
You have one or two system vSphere Distributed switches for system traffic and a separate vDS for overlay traffic. MultiDvsAutomator script