Answer
This is a framework limitation, not something missing from your setup — Custom Tabs can only be attached to Tab Form windows (existing standard application windows like Purchase Order or Customer Order), and when creating a Custom Page, your only choices are a Table Window or a Form Window type — there's no 'Tab Form' custom-page type available, since it isn't supported by the IFS Framework. Because of this, Custom Tabs simply can't be added inside a custom page, regardless of how it's configured; that combination isn't something the framework allows.