ESXi and PowerCLI

I have created a 2 node vsan cluster with a witness.  The two physical hosts are interconnected with 10gb links and the vsan vmkernel resides on that particular vswitch.  I leave the mgmt on the default vSwitch0 which uses vmk0.

I'm looking for a way to script in powercli/powershell adding a route to the esxi hosts so that it can communicate with the witness.

$HostNameFQDN = xyz.domain.com

This outputs nothing on my system currently but it seems like i'm really close.
$Host_Gateway = Get-VMHost -Name $HostNameFQDN | Get-VMHostNetworkAdapter | select VMKernelGatway

New-VMHostRoute -VMHost $HostNameFQDN -Destination 172.23.0.0 -Gateway $Host_Gateway -PrefixLength 20

Would somebody be able to assist with getting this to work, correctly?  I've also noticed that the Get-network commands do not work in Powershell ISE but they do in the regular powershell window.

Thanks in advance.