How to bend a 3D object in HFSS

I don't think you can. What I do in these cases are create an equation-based line, then one can assign a cross-section such as a rectangle.

Curved objects can be created in a variety of ways. Boolean operators working on primitives is one method. One can also sweep surfaces along lines. A helix is an example. I have on occasion wanted o sweep a solid along a path but unfortunately that ability is not in the modeler. Maybe they will add it someday.