Selectively route VPN traffic - how do I determine subnet value?
You don't need to create a new subnet, you just need to figure out what subnet (or subnets) your company's network uses. For example, if the IP addresses on your company's network are in the ranges 10.0.0.1 to 10.0.0.255, then you can use the subnet mask "10.0.0.0/24". If the range is 10.0.0.1 to 10.0.255.255, then you can use the broader range "10.0.0.0/16". You can search for "subnet calculator" to find some tools that can help you figure out the notation for the subnet range you want to capture.

Make sure your local network doesn't overlap with the range, though. Your home network should use a different subnet (like 192.168.0.0/24 for example) that won't conflict with your office's network.