You would define the method in the cusPages.prg and use the INIT of the pgNames to call it. OR...
If you have added the pageFrame to the form and you know that this page will exist, then you can call it from there.
I would suggest the first method. Keeps your code in one place. Also it is nice to have all of these special page definitions in a single place. When you move to VFP8, I am sure you will be able to convert the code you created into the visual classes.