You want to add Custom Fields in APP Reassign Costs and Fields, for example, you want to add Custom Fields to the Items Line.
SAP S/4HANA Cloud
Currently, APP Reassign Costs and Revenues does not support Extensibility and Custom Fields during posting.
Please raise feature request according to KBA 2963059 - How to Submit Feature Requests for SAP S/4HANA Cloud Product.
Custom Fields, Custom Fields and Logic, Reassign Costs and Revenues. , KBA , CO-FIO-CCA-TRA , Transactions , Problem