How to put electric field inside a detector using root file?

I am a newbie to the Geant4 simulation.
Can anyone please provide me how to put an electric or magnetic field inside an HPGe detector as a “#.root” file? I have the electric field inside an HPGe detector in a root file but I don’t know how to put it in a virtual detector via Geant4 by constructing “” and “#.hh” files.

Any kind of user-friendly documentation or any kind of article will be sufficient.

Thank you.