If you have installed the onyx profiles for the FB500, you will have profiles for CMYK, for printing white over color or white under color.
If you want to let onyx just generate a white layer you need to open job editor, go to the tab where you do color replacement, and select the spot color layer option (not shure if this is the correct name, use german onyx). There you can select if Onyx underfill only printed areas, white areas and so on.
If you want to print white spot color, you should color the objects you want to print white in a spot color named „Spot1“ in your design software. You can use different color names, but you dont need to set up „Spot1“ in advance separately, is pre-defined.
If you want to underfill or overfill selected areas, you need to draw the areas you want to print white, and set the white areas and the objects they should underfill to „overprint“ (Illustrator in the attributes).