Answer
Write a custom PL/SQL function, following the same signature convention Object Connection Transformation variable functions require (accepting a target key reference and a service name, returning a VARCHAR2 key-reference-formatted string), and inside it look up the actual value you need dynamically — for example calling the Technical Object Reference API's method for getting a technical class by key, against the part number extracted from the target key reference, rather than returning a fixed literal string. Reference that custom function as the transformation rule's variable source instead of the hardcoded constant, and it resolves to the correct value per record at transformation time.