Electric field only for selected atom/s

GROMACS version:2020.4
GROMACS modification: No
Dear gromacs users,
I am trying to run an md simulation with an oscillating electric field where the electric field only applies on a selected group of atoms in a certain direction, not to the whole system. It seems like the gromacs electric field is not designed in that way. When I apply the electric field, it is for the whole system. I am aware that the phenomenon is not natural but I really like to test my hypothesis. Could one please suggest to me a way to apply the electric field only to one atom or a small group of atoms?
Thanks,
Suneth.