Add the route details in the below file to make it persistent across reboots.
# /etc/systemd/network/XX-eth#.network
Where X can take any number depending on your system and # is the number of the interface you want to add the static route to.
Add a new section at the end of the file in the format as below:
[Route]
Gateway=10.20.30.40
Destination=192.168.0.0/16