With "Generate Support" disabled, untick "Remove Empty First Layers" under "Mesh Fixes".
Alternatively: 1) Adjust the high. 2) Click "Support blocker" (sixth icon on the left) click anywhere to place it. 3) Select the cube, click "Per model settings" (fifth icon on the left) and click "Normal model". 4) Click "Select settings", select "Wall line count", "Top Layers", "Bottom layers" and "Infill density", set all of them to 0 and slice you model.