Create a material with the correct magnetic and electric material properties. Select a surface where the thin film will be applied and apply a layered impedance boundary condition. Enter the thickness and the user defined material along with the substrate material, ie copper. It is of note that HFSS can only support linear magnetic material...which is usually fine for ferrites, but I do not know for your material...ensure there is no significant variance in the B-H curve for the current magnitudes of interest.
have fun