From what I can see, this field is not exposed in the DI API. From what I can see in this thread, it looks like it might have just been added in 9.1: How to handle wastage in production process of SAP Business One?
I have 9.0, so I cannot confirm this, but usually when a new field is added into SAP, the field is not immediately exposed in the DI API unless there are service calls/requests to expose it by SAP Partners.
So, unless you have an updated version of the DI API that I don't have (my version is again 9.0), I think you might be out of luck and should contact SAP directly to request this feature in the next version if you need exposure to it through the DI API. Otherwise, you're stuck with dealing it on the form itself through the UI API using tricks, or updating the field with a direct SQL call, which is NOT recommended and may void your software warranty.