Once you have chosen to Add Cabinet/Panel in your existing room.

Underpanel is found in Wall -> Panels & Fillers.

Enter the Width and Depth that the Underpanel is meant to be.

NB. Height of Underpanel is the thickness of the material and the Depth varies, depending on if the panel is going to protrude past the doors, sit behind the doors or provide a gap for finger pull.

Close dialog box to complete entry.

Once you have completed this item and entered any other items that may require ordering, to the room, then follow the process to order a room.