Having trouble adding potion effects to mobs

Hi, I'm having trouble adding a potion effect to a zombie. After an hour of searching on Google, I still can't figure it out.

Here's what I have

Code:

public void event(CreatureSpawnEvent event) {
        if (event.getEntityType() == EntityType.ZOMBIE) {
            LivingEntity zombie = event.getEntity();
            zombie.getEquipment().setHelmet(new ItemStack(Material.DIAMOND_HELMET));
            zombie.addPotionEffect(new PotionEffect(PotionEffectType.INVISIBILITY, 999999, 1, true));
        }
    }

All the zombies spawn with a diamond helmet, but I can't get the potion effect to apply.

 

That is because the event code gets handled before the mob actually spawns.
Schedule a task 1 tick in the future to add the effect to the mob, once it has spawned.

 

I just tried for an hour to get a runnable to work for this, do you have any examples I can work off of?

 

@Theztc
https://letmegooglethat.com/?q=bukkit+schedule+task