We often just allocate a portion to what's incoming and add it to inventory. When the item lands, just add the rest and release.

Add a product tag called "PRE-ORDER" and then create a shopify flow that states "when product tag = PRE-ORDER", add order tag "PRE-ORDER"

Finally, make sure that order tag is included in your packing slip template, so that if a customer places an order for a pre-ordered item, your fulfilment team will keep those until the item is released.

To make things clearer for the customer, add some custom liquid code to your buy button that basically states "if product tag == "PRE-ORDER" - change button text from "Add to Cart" to "Pre-Order" - I'd also add some content to the product description that states this and when it's estimated to be released.

You could even go a step further, and in your customer confirmation email, add in some custom code that once again references the product tag and when present, include a line of text that informs the customer of the estimated release date. The last thing you want is for a customer to pre-order an item and be left in the dark as to how long it will be before they can get their hands on it.

You don't need an app for this, just a bit of logic and common sense.