MUI2 - Page Init function

Good morning, it is a fault of mine for sure, but I can't understand from the documentation and the examples how assign a callback to a page when using MUI2. Can someone tell me where to read or how to do it?

Thank you in advance.

...
!define MUI_PAGE_CUSTOMFUNCTION_PRE mycomponentsprecallbackfunction
!insertmacro MUI_PAGE_COMPONENTS
...

Thank you very much!