public SignedDocumentDelivery AddDestination(External value)
        {
            if (value == null)
            {
                throw new ArgumentNullException("Argument cannot be null");
            }

            _destinations.Add(value);
            return(this);
        }
     public SignedDocumentDelivery AddDestination(External value)
 {
     if (value == null)
     {
         throw new ArgumentNullException("Argument cannot be null");
     }
     
     _destinations.Add(value);
     return this;
 }