Defining boundary condition in electrostatic solver in Comsol

Hi
I need to solve the bellow 3D problem in comsol
I have a periodic metallic square patch arrays located at xy plane with 1.8mm width and 2mm period. we can use a unitcell ontains only one square patch with periodic boundary conditions in x and y directions. I need to calculate static electric fields in all directions in all the space.
the only boundary condition is Ex=-1 (tangantial component with respect to the surface) on the top and bottom surfaces of the patch. I can not define this boundary condition, because available boundary conditions are only Neumann and Dirichlet boundary conditions with this formula d/dn and n.E (normal component to the surface), so I can not impose any tangential boundary conditions!

can anyone help me?