In each order in the Awaiting Shipment page, there will be an Allocate button if that order has not been fully allocated (to the fulfillment process), you click on that to start the process.
After you click on Allocate button, the Order Allocation popup will show.
Then you select the Allocate Type = Back Order. All items still need to be allocated will show on the popup.
You can select the Backordered Qty of the items and click on Add Request button to create a Backorder for that order. (note that there may be some ordered qty has been put into Backorder, and they cannot be allocated until you send them back to fulfill).
After the backorder request is created successfully, a toast message will show “You have created a backorder request successfully!”.