function CreateCustomPage(const AfterID: Integer; const ACaption, ADescription: String): TWizardPage;
Creates a custom wizard page. The page is empty by default; you have to create your own controls afterward and place them on the page (by setting their Parent properties to the Surface property of the TWizardPage instance returned by this function).
See CodeClasses.iss and AllPagesExample.iss example scripts in the Examples subdirectory in your Inno Setup directory for examples.