Code required for selective network routing. Check out the blog https://rohanrajpal.github.io/workfromhome/vpn/networking/selective/routing/systemd/2020/04/25/Selective-network-routing.html

my-pppscript.sh

#!/bin/bash
#
# Whitelist here all domains that need to go through openfortivpn
# Domains and IPs are separated by a space
#
ips='192.168.2.217 192.168.29.151'
domains='example.com example.fr'

let resolved
for domain in $domains; do
  resolved=`dig +short $domain | tail -n1`
  ips="$ips $resolved"
done

for ip in $ips; do
  route add $ip dev ppp0
done

openfortivpn.service

[Unit]
Description = OpenFortiVPN
After=network-online.target
Documentation=man:openfortivpn(1)

[Service]
Type=idle
ExecStart = /usr/bin/openfortivpn -c <path to your config file>
StandardOutput=file:<any-place-where you want to save your logs>
Restart=always
RestartSec=10

[Install]
WantedBy=multi-user.target

vpn-config.conf

host = vpn.iiitd.edu.in
port = 10443
username = <your username>
password = <your pass>
set-routes = 0
set-dns = 0
pppd-use-peerdns = 0