Get a Instock Notifications Collection and use LINQ to locate all notifications with a ProductCode matching what you specify and log the email address of the user owning that notification.

instocksnippet.cs

var instockResource = new Mozu.Api.Resources.Commerce.InStockNotificationSubscriptionResource(_apiContext);
var notifications = instockResource.GetInStockNotificationSubscriptionsAsync().Result;
var notificationsForProduct = notifications.Items.Where(x => x.ProductCode == "MS-BTL-002-1");
foreach(var notification in notificationsForProduct)
{
  Console.WriteLine(notification.Email);
}