If you only have 3 Axis, then you would need to do it in 3 operations. Chamfer the top at the highest Z level, then turn the part on it's side to finish the cutout part that had the lower level Z position where the slot is. This would give you a perfect chamfer depth and angle all the way around the circumference.
Use HSS user defined projection. Select the surface, select the upper or lower curve and a tool big enough to do it. I usually use a ball mill for chamfers like that.