Route specific IP through OpenVPN
I forgot the interface must be an existing one, so I used utun which is on macOS by default.

I removed all entries for route in the OpenVPN config file and instead installed iproute2mac via homebrew, brew install iproute2mac so I have access to the ip command which I am used to the syntax of.

I then ran ip route add 42.123.123.10 dev utun2 (I know my interface will be utun2 as without OpenVPN running there are two utun interfaces for me, utun0, and utun1 checked with ifconfig or ip r.

Now it works.

OpenVPN allows you to run scripts via the configuration files so I will dump these route add and route delete rules in some files.