There are two areas where customers can make a purchase on your web store: the open storefront and the allocation page. The open storefront displays products to a customer whether or not they are logged in, and might also display allocated products. Follow these steps to remove the "add to cart" for an allocated product and enter a custom message directing your customer to the allocation page:
Navigate to the Products tab
Search for and open the allocated product
Click the monitor icon (Edit display details) in the upper right.
Scroll down to Allocation and check the box next to, Treat this product as allocated for display purposes.
Enter a custom message in the Allocation Message box. This field accepts HTML. Example:
This product is allocated to our mailing list customers. <a href="/account#allocations">Click here to view your allocation</a>
After saving your changes, be sure to look at the storefront on your website to see how the allocation message looks and test the link to make sure it works.