Place By Flood Slab Placement

AECOsim Building Designer SS6. Version -

Hi All,

Can someone please demonstrate to me how i can convert the hatched region illustrated in the image below into a slab (1300mm Thickness) particularly using the "place by flood" option. 

I have used the "create region" tool to define my region but whenever i try to flood the newly created region, it doesn't recognize it.

Meanwhile, I welcome any other suggestions for  achieving this slab form as long as it can be listed in the dataset explorer for scheduling/reporting

Thank you