How to add an extra loopback IP address on your MacOS

extra.loopback.alias.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">
<!-- copy to /Library/LaunchDaemons/extra.loopback.alias.plist -->
<plist version="1.0">
    <dict>
        <key>Label</key>
            <string>Loopback Alias</string>
        <key>ProgramArguments</key>
            <array>
                <string>/sbin/ifconfig</string>
                <string>lo0</string>
                <string>alias</string>
                <string>127.0.0.3</string>
                <string>netmask</string>
                <string>255.255.255.0</string>
            </array>
        <key>RunAtLoad</key>
          <true/>
    </dict>
</plist>

Copy to

/Library/LaunchDaemons/extra.loopback.alias.plist

and reboot or load it with launchctl load /Library/LaunchDaemons/extra.loopback.alias.plist