All SnapEDA parts have slots represented on layer 46 Milling, this is the way Eagle recommends doing it. When you go to generate the Gerber you will generate a separate Gerber file containing only layer 46. You will tell your board house that the info on that Gerber needs to be milled from the board.
If your board has internal copper layers then you need to isolate the slot from them manually. You can use cutout polygons for this purpose. If you run the autorouter you will have to make sure that none of your traces run through the slot.
If you are using OshPark, you may refer to this blog here on how to define cutouts and slotted holes in Eagle.