You can add instances for monitoring in DMSO through
The user interface
Add an instance to the Management Cluster which is current at the time of addition.
To determine which repository is used, you may need to allow the current value of the ActiveRepositoryConnection property
in the DesktopClient.Properites.Settings class.
A new cluster-aware PowerShell Cmdlet
Check the example below of PowerShell Cmdlet for adding an instance for monitoring:
New-SQLdmMonitoredInstance -Path Instance1 -WindowsAuthentication -tags production,finance,critical
Also, specify the Management Cluster in addition to the Instance name. Check the following example:
New-SQLdmMonitoredInstanceClustered -Path Instance1 -Cluster MgmtCluster -WindowsAuthentication -tags production,finance