how to create sub domain in ansys
 

good afternoon to all, i need help to solve the below problem which is a part of my project.
i am working with ansys polyflow- blow molding. for calculating the pure mass of the material which is blown in the cavity, ansys says that i need to define a sub domain which shows just inside the cavity, not other parts of the cavity. how can i define a sub domain for a specific part of a cavity in ansys?

hope to be helped,
thanks all.