Just so you can paste it in the terminal and it’ll be done and you won’t need to use vi/nano to create the files and paste the content there manually.
You don’t need to run it after the specific guest is started. You can just start it after WantedBy=multi-user.target. If the guest is not running and the service is started nothing bad will happen, it’ll just refuse the incoming connections from dom0, but won’t fail or anything.