How to add a mass source term to NS equation

Hello everyone

I have a problem in adding a mass source term to the NS equation. I use it to account the mass loss due to vaporization during laser melting process. Currently I use the interface moving speed based on mass flux in the moving mesh module. But I'm not sure if it works properly.

Previously in V3, I can easily add the source term to the NS equation in the equation system.
But in the current V5 or V4 COMSOL, I cannot find the equation system, or any method to add a weak contribution to the continuity equation.

Do you have the solution? Thank you very much for your help.