There is no custom function feature in LoadUIWeb. It's best to stick to "hand-off" style operations in a load testing tool, and leave the conjugation processes up to the server (in source code).
While in nominal circumstances this seems useful, it's really easy to over-use this kind of functionality (as I have seen many people do in other tools) and thereby introduce unrealistic skew into test results unknowingly. People tend to start with simple string manipulations, but then either the developers change something and break the testing custom functions, or the function performs mutable operations (on non-immutable data structures) that drag the results down when run on mass scale.
