This calculation is done the same way as any other mode. OptiFiber does not distinguish between cladding modes and core modes. It just finds modes. When you look at the mode field pattern, you may find the majority of power of some of them to be in the cladding, rather than the core. That’s a cladding mode.
In order for the cladding mode to appear, you need to make a layer in the simulation having the width of the cladding. You also need to include another layer outside the cladding, consisting of air or whatever material is found there. The refractive index of that outer layer is usually much smaller than glass, so a thin layer will suffice for the simulation.