/usr/share/doc/webgui/examples/preload.custom.example is in webgui 7.9.33-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | # Add paths to lib folders where you have custom plugins for WebGUI.
# Note that the folder must contain the same directory structure of
# of WebGUI itself. This file should reside at WebGUI/sbin/preload.custom
/data/Custom/lib
|