how to write boundary function file for flux defined as J = B(hs −henv) in parabolic pde solver?

I want to write a function file for flux defined by J = B(hs −henv) in parabolic pde solver.

where, hs is the humidity at the boundaries of a square, henv is the humidity in the environment i.e., a function of time and B is the surface chloride transfer coefficient (a constant).

Thank you for your help!