I tried using RegistryEvent.MissingMappings and it won't work, where did I go wrong

 @SubscribeEvent
    public void missingMapping(RegistryEvent.MissingMappings<Item> event) {
        for (MissingMappings.Mapping<Item> entry : event.getAllMappings()) {
            if (entry.key.toString().equals(DragonMounts.MODID + ":ender_dragonscales"))  {
                ResourceLocation newTotem = new ResourceLocation(DragonMounts.MODID + ":ender_dragonscales1");
                entry.remap(ForgeRegistries.ITEMS.getValue(newTotem));
            }
        }
    }

I renamed an item fro an experiment, I clearly have no idea how to use RegistryEvent.MissingMappings