The FME Transformer Designer allows developers of FME Packages to build a graphical user interface for FME Transformers they develop in Python. For details on how to create FME Packaged Transformers, refer to the FME Packages SDK.
The FME Transformer Designer creates .fmxj files, which capture the transformer interface and can be read by FME Workbench. Refer to the Package project structure documentation in the FME Packages SDK for details on where to store the .fmxj within an FME Package.