I am not sure if I understand what you have done.
Split the body with XY plane. Then you will get 2 bodies. Hide or delete one body. Then select the surface at XY plane and use ctrl + c and ctrl + v to copy paste that surface. Then you can hide or delete the solid body. The copied surface is your 2D geometry. Make sure you have it in positive XY plane.
Please go through help manual for more details
How to access Ansys Online Help Document
Guidelines on the Student Community