It's not recommended to use essential oils in a warm mist humidifier since warming the essential oils with a warm mist humidifier will damage the therapeutic properties of the essential oils.
If you do use a humidifier with the essential oils you should use a cool mist humidifier instead.
But it's best to use a diffuser for the essential oils since the diffuser will break down the essential oils into tiny particles that are more easily absorbed and inhaled.