Answer
Getting incoming custom-field Attribute data actually applied to the created order requires PL/SQL development work on the receiving side — specifically, implementing the stub methods intended for this purpose (named something like Add_Order_Attribute and Add_Line_Attribute) in the relevant order-service utility API package. These stub methods exist specifically so custom-field handling can be implemented per customer's specific custom fields, but they don't do anything by default — the web service's Attributes structure alone isn't sufficient without that implementation work, which typically requires either your own IFS development license or engaging your IFS consulting partner to implement.