Unfortunately there is no New-ExternalNetwork, only a New-OrgNetwork cmdlet. It should be possible as there is a cloud view called VMWExternalNetwork, but I've never used it before, but It would start like this:
$mynetwork = new-object vmware.vimautomation.cloud.views.VMWExternalNetwork
$mynetwork.Name = "External_Network_1"
$mynetwork.Description = "My First External Network"
$mynetwork.VimPortGroupRef = new-object VMware.VimAutomation.Cloud.Views.VimObjectRef
$mynetwork.VimPortGroupRef.VimServerRef = new-object VMware.VimAutomation.Cloud.Views.Reference
$mynetwork.Configuration = new-object VMware.VimAutomation.Cloud.Views.NetworkConfiguration
$mynetwork.Configuration.IpScopes = new-object VMware.VimAutomation.Cloud.Views.IpScopes
$mynetwork.Configuration.IpScopes.IpScope = new-object VMware.VimAutomation.Cloud.Views.IpScope
Then you will have you initial building blocks for your external network. If you can't puzzle together the rest you may need one of the VMWare guys to assist or Jake Robinson is pretty good with Cloud Views. Sadly there is slim to no documentation for the all powerful cloud views.
Cheers!