It is no longer possible to create "undefined page link" in Confluence Cloud, since the introduction of the new Fabric Editor.
- Create new or edit page in fabric Editor
- Insert Hyperlink (/hyperlink)
- Type name of new Confluence page you want to create (undefined page)
- Save page
Clicking on the link to undefined page creates the new page with link title and opens Fabric Editor and user can start entering contents to new page.
Clicking on the link brings up a blank page (about:blank).