Configure Newly Added to Pool in Drain Mode

andrewasdr Think this might help get you going.

First add the VMs to the output of your deployment template, I use this in my ARM template:

"outputs": {
        "virtualMachines": {
            "type": "array",
            "copy": {
                "count": "[variables('virtualMachineInstanceCount')]",
                "input": "[reference(resourceId('Microsoft.Compute/virtualMachines', concat(variables('virtualMachineNames')[copyIndex()])), '2020-06-01', 'Full')]"
            }
        }
    }​

Using this output, from the New-AzResourceGroupDeployment cmdlet, I populate an array with the VM resources and update the session hosts to not allow new sessions.

$vmResources = @()

foreach( $vm in $deploymentResult.Outputs.virtualMachines.Value ) { $vmResources += ($vm.toString() | ConvertFrom-Json).Properties.osProfile.computerName }

foreach ( $sessionhost in $vmResources ) {
    $sessionHostName = "$($sessionhost).domain.local"
    Update-AzWvdSessionHost -HostPoolName $hostpoolname -ResourceGroupName $rsgName -Name $sessionHostName -AllowNewSession:$False | Select-Object Name,AllowNewSession
}

 The Select part at the end is just to beautify the output, since Update-AzWvdSessionHost returns the updated record and by selecting the Name and AllowNewSessions you get a nice table showing all the session hosts that have been updated.

Hope this helps!