Random distribution on polysurface + voronoi tassellation

Hello everyone I'm Dario, "a new follower of the grasshopper"... I'm trying to make a random point distribution on a poly surface from which to derive a voronoi tassellation...It's simple with a single surface but reather complicated with a poly surface...could you help me?...I can not get the pictures(upload problems), then insert the same location as the reference files...Thanks