a question on the chemical potential in fix ccmc

Hello! I’m using 2016 version of LAMMPS. I have a container with particles with a nano channel on one side through which these particles are leaking. I’d like to use “fix gcmc” in such a way that the number of particles inside the container stays constant although some are leaving through the nanotube. Therefore, some particles must be added with fix gcmc. The question is - how do I set the chemical potential?
Thank you,