CST - Specify the Plane dimension for Power distribution Plot

Dear Altruist,

I am trying to extract the power distribution plot on a plane across different depths of the designed structure in CST studio suite. When I am selecting Fields on Plane it is creating a plane but it is not giving option to specify the plane.  I want to create a specific plane lets say -20 mm to 20mm on which I want to see the power distribution across different depth of the designed structure. please help.