Default value in DFF based on user selection of standard field

Core Business Requirement : Based on the standard operation selected on the maintenance work order, one of the segment of COA needs to be updated.

Issue: There is no standard input source related to maintenance standard operation which can be leveraged to derive the mapping set.

Alternative workaround : We have created a DFF at the Work Order operation level and with help of table type value set trying to default value on the DFF field. Further based on that trying to achieve the above requirement to derive the right accounting segment.

Help Required: This DFF at work order operation level, should auto-default at save based on the standard operation selected on the document. We are facing issue to auto-default the value Or suggest any other workaround to achieve the above requirement.