Implementing non-standard boundary condition

I am trying to solve a standard diffusion pde in potential form. The resulting equation consists of a term with a first-order time deriviation and another term with a second-order space deriviation. The flux at the boundary becomes then also a second-order deriviative by space.
I found a way to define the pde using the "Transport of Diluted Species" model. However, I do not manage to implement the bc. Is there a way to implement my second-order space deriviation BC in a user-defined manner? Or any suggestion which Comsol model I should use?
Cheers, A