How to perform manual item substitution on a Sales Order using an API call?

We are currently preparing a Sales Order extension where users can have the ability to do mass updates on sales orders. One of the updates the user can do is to add a substitute item if they want to change the original item. (This substitution should be manual and not automatic)

As most of orders are imported to OM, we used the CreateOrderAsync service to import an order to Oracle.

Once the order is successfully processed and in awaiting shipping status, we run the "Update scheduling attributes" POST API call to substitute items on the fulfilment line (/fscmRestApi/resources/11.13.18.05/salesOrdersForOrderHub/action/updateSchedulingAttribute)