Select the drawer you want to change to tandem box.

Look for the menu labeled "Drawer" on the right of the design panel. It would be arranged in a vertical order as Cabinet>Partition>Drawer Systems>Drawer> Panel etc. Click on it to access the settings related to drawer.

Within the "Drawer" menu, go into the "Quick Access" or "Construction" tab and you will find the "Drawer Accessories" section. Click on the drop down next to it to open the details.

Under "Accessories Type" select the option "Tandem Box", which will open the catalogue showing the relevant offerings in your catalogue.

Select the required option with the correct product specifications and click on the same to change that drawer into a tandem box.