The following PowerShell commands can be run on the SCVMM server to add an existing Azure Stack HCI cluster to VMM 2019.
$runAsAccountName = 'Administrator'
$runAsAccount = Get-SCRunAsAccount -Name $runAsAccountName
$hostGroupName = 'S2DHosts'
$hostGroup = Get-SCVMHostGroup -Name $hostGroupName
$hostCluster = 'R740Cluster.hci.lab'
Add-SCVMHostCluster -Name $hostCluster -RunAsynchronously -VMHostGroup $hostGroup -Credential $runAsAccount -RemoteConnectEnabled $true
Once the job completes, verify that the cluster and cluster nodes are available in VMM.
Get-SCVMHostCluster -Name 'R740Cluster.hci.lab'