There is one example of custom anonymization function in lib/functions.cpp when creating sample project:
This particular function is written in C++ and is country specific, it is a citizen ID in Czechia and Slovakia. The number need to confirm some rules, so you need to anonymize such values unusual way to keep the validity of such number.
You can use such function as an example and add your custom C++ codes into lib/ project subdirectory.
Or once you consider that your custom function would be useful also for others, or you need help with C++ code, do not hesitate to contact us at firstname.lastname@example.org.