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

Hi
I use Electrostatic mode for the solving Maxwell equation and PDE mode for the solving
conservation of space density,
but I don,t know how I couple these equations, because for default,
COMSOL suppose space charge density=0 in maxwell equation, while
charge density is not constant and solve with coupling of Maxwell and
conservation of space charge density. Please help me if possible.

Thanks for your attention