To enable the pre-order button, you can simply navigate to the online store --> themes --> Xpert --> customize --> theme settings --> Preorder. From there, they can enable the pre-order button and customize it.

Hide the inventory text from your Product detail page and Collection pages for all PreOrder products. Just uncheck "Show inventory for pre-order products" and it will be removed.
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Furthermore, you can add a delivery label to show messages or you can use metafields to display the content you want to show to your customers:
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To access delivery label area, you will need to open the product page template > look for buy now button and click on it.