Something similar has been discussed (such as at Continue interrupted backup option - #3 by JonMikelV) but I don’t believe anything has been done to implement it.
Until that feature is added some options to do what you want include:
- schedule more frequent backups (such as hourly)
- use Windows Task Manager to set up a trigger script as described at Device mount detection (USB or otherwise) - #5 by Beller0ph1