Begin by selecting the cabinet where you want to add a countertop ply.

Under "Cabinet" Menu, look for the section labeled "Add accessories" on the right of the design panel. It will be available both in the "Quick Access" and "Customise" tabs.

Under that you will find an option "Add countertop ply" which when you select, a countertop ply will be added between your cabinet top panel and the countertop.

You can select the panel added and go into the "Panel" menu now to make changes like materials, sizes etc to it.