pde elliptical – Solution of the Laplace equation in an isosceles trapezoid

I'm new in this forum. I hope someone helps me with this problem.
Therefore, I have to solve the Laplace 2d equation (Δu = 0) on an isosceles trapezoidal domain. line AD has a slope 1 and line BC has a slope -1. Please see the png file.

My problem is, how should I put the boundary conditions for lines AD and BC? For a rectangular domain, it is easy to insert the limit values, since all sides are 90 degrees apart. but in a trapezoid for the sides without parallel I have no idea. Please, help me find out.