First, you need to follow Shopify documentation to setup preorder for our products. Once your products are set up for Pre-Order, our app can help you get subscriptions for Pre-Order products.
For example, you have a product thats in Pre-Order/Coming Soon state. You would like customers to sign up for it so that they get notification when product is in stock. That's where our app comes in picture.
Note: You must be on any paid plan.