Enable preorder and backorder functionality
You can use the updateInventoryConfiguration endpoint to
    enable preorder and backorder functionality. 
               
By default, preorder and backorder functionality are disabled in Commerce. This section describes how to use the updateInventoryConfiguration
      endpoint to enable preorder and backorder functionality. Preorders and backorders are enabled
      and disabled at the same time with the preorderBackorderEnabled property. You
      cannot enable or disable them separately.
               
The following sample request enables backorder and preorder functionality:
PUT /ccadmin/v1/merchant/inventoryConfiguration HTTP/1.1
Authorization: Bearer <access_token>
{
  "preorderBackorderEnabled": true
}