Add static route at startup

Ah, that will cost extra! ;)

Barney-15E hit the nail on the head. That is you will want to load this script on boot by using a LaunchDaemon.

For example:

• Start by creating a new LaunchDaemon file in: /Library/LaunchDaemons
• To do so, enter the following command in the Terminal app: sudo nano /Library/LaunchDaemons/com.custom.staticroute.plist
• Enter the following into that plist:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
  <dict>
    <key>Label</key>
    <string>com.custom.staticroute</string>
    <key>ProgramArguments</key>
    <array>
      <string>/etc/rc.local</string>
    </array>
    <key>RunAtLoad</key>
    <true/>
  </dict>
</plist>

• Save, and then, load the LaunchDaemon with the following command in the Terminal app: sudo launchctl load /Library/LaunchDaemons/com.custom.staticroute.plist

"static route"?

"Static Route" as in configuring a specific network route for your Mac to take when communicating to the Internet?

You could create a LaunchDaemon plist to run your script.

You would not need sudo as a launch daemon is run as root.

https://www.launchd.info/