$rgName = "tmp"
$ssName = "ss name"

# Get information about the scale set
$vmss = Get-AzureRmVmss `
                -ResourceGroupName $rgName `
                -VMScaleSetName $ssName -Verbose


# Add the Custom Script Extension to install IIS and configure basic website
$vmss = Add-AzureRmVmssExtension `
    -VirtualMachineScaleSet $vmss `
    -Name "customScript" `
    -Publisher "Microsoft.Compute" `
    -Type "CustomScriptExtension" `
    -TypeHandlerVersion 1.8 `
    -Setting $customConfig 

# Update the scale set and apply the Custom Script Extension to the VM instances
Update-AzureRmVmss `
    -ResourceGroupName $rgName `
    -Name $ssName `
    -VirtualMachineScaleSet $vmss -Verbose