How to Enable Pre-Order in Shopify Horizon Theme

Pre-order is a native Horizon feature, but it's driven by your inventory settings first — the theme just displays it correctly once the underlying setup is done.

Steps

In your Shopify admin, go to the product you want to offer as pre-order, under Inventory.
Turn on Continue selling when out of stock — this is what actually allows a purchase before stock arrives; it's a store-wide product setting, not a theme setting.
In your Shopify admin, go to Online Store > Themes and click Customize on Horizon.
Open that product's page and check the buy button block — Horizon should automatically show pre-order-style messaging when a product is out of stock but sellable.
If Horizon offers a custom pre-order label or messaging setting, set it there.
Click Save.

Without "Continue selling when out of stock" turned on for the product, Horizon has nothing to display — the theme setting alone doesn't enable pre-order sales.