To add a permanent static route in MacOS Mojave, open up a terminal session.

List all of your available network services. Services really just seem to be aliases for network interfaces.

$ sudo networksetup -listallnetworkservices
An asterisk (*) denotes that a network service is disabled.
USB-Serial Controller D
iPad USB
iPhone USB
Bluetooth PAN
Thunderbolt Bridge

You will be adding the route to a service, therefore, you will need to know which service to apply the route to.

In my case, I wanted to add the route to my wireless interface:

$ sudo networksetup -setadditionalroutes Wi-Fi network-destination destination-mask gateway-to-use
$ sudo networksetup -setadditionalroutes Wi-Fi

List all of your current routes:

$ sudo networksetup -getadditionalroutes Wi-Fi

To remove static routes from a network service just remove the parameters from the command you used to set the route:

$ sudo networksetup -setadditionalroutes Wi-Fi