Problem with Neumann boundary conditions for discontinuous Galerkin method

I am trying to impose flux boundary conditions (Neumann type) in the 2D heat conduction equation. The int1d(Th,3)(…) works fine for the Galerkin method, but this integration does not affect boundary values in discontinuous Galerkin.
Please help me to impose the flux boundary condition for Discontinuous Galerkin in FreeFem++.