You don’t need to!
TopShape will automatically generate the appropriate elements inside of solids before doing analysis and optimization.
Like CAD software, and unlike conventional FEA software, TopShape only let you work on the surface representation of solids. In fact, you can never physically touch the inside of a solid, so there is no need for users to interact with solid elements at all. Topshape will try to mesh the enclosed volume with Hexahedral elements if possible. If not, it will mesh using Tetrahedral elements.