How we solve electrohydrodynamic eauation(Maxwell and Conservation of space density) in COMSOL?

Hi
I use Electrostatic mode for maxwell equation and pde mode for conservation of space density,
but I don,t know how I couple these equation, because for defult, comsol suppose space charge density=0 in maxwell equation, while charge density isn't constant and solve with couple of conservation of space charge density. Please help me if possible.

Thanks for your attention