Want to generate a segment on charge account and accrual account using attribute Customer on SO

Summary:

Depending upon customer name in Sales Order we want to update the Trading Partner Segment in Charge Account and Accrual Account. We are thinking of approach to create DFF on PR and PO and via help of manage Service Mapping will automatically populate the DFF on Dropship PR and PO as per the customer Name on Sales Order. And will use this DFF to generate Trading Partner Segment in Charge Account.

Please can you help me in creating Service Mapper Setup as per the implementation guide I'm trying to create the setups but implementation guide is related to Custom EFF and I need to update customer data on PO header DFF and not able to understand which specific entity, source and service I should select and which attributes needs to be selected it should be custom as mentioned in guide or can use any seeded.