how to render line between gui element and mouse position

i am trying to use OpenGL to render line between gui element and mouse position.

i am using this:

    GL.Begin(GL.LINES);
     GL.Color(Color(1,1,1,1));
     //GL.Vertex3(10,10,1);
     GL.Vertex3(Input.mousePosition.x,Screen.height-Input.mousePosition.y,1);
     GL.Vertex3(100,100,1);
    GL.End();   

and it does nothing, if i use the commented line instead the line with mousposition i get the line but in 3d space, this isnt what i want and i guess that it has to be something with screen space... can somebody help me how to do it?