Primitives are simple 3D objects such as cubes, spheres, planes and cylinders. Most 3D applications can create them – but Poser cannot. Therefore, trying to hunt for a menu such as Object – Create Primitive will be in vain.
Hence, you cannot create a primitive in Poser. At least not that I know of.
There is good news though: Poser’s default content contains plenty of them, so all we need to do is load one into our scene. You can find these in the Library, under Poser (your version) Content – Props – Primitives.
If you need anything specific, it looks like you’ll have to create your own primitives in another 3D modelling app.