I dont have etabs right now. Hollow core slab is a one way slab. Therefore, in order to model thr one way behaviour, u display the local axes of the area and u assign 0.01 to the direction perpendicular to the main direction of the hollow core. The use of the 0.01 is just for the slab to act in one way.
As how to apply it, u select the slab go to assign and click on property modifiers and input the values.