You can enable authentication for your app via the Auth section in the global toolbar.
When you choose an authentication type, the relevant pages are installed for your app.
Initial view is a fullscreen page that is opened first, before the user is given access to the rest of the app. In most cases, it is used to show a login form.
When your app has authentication enabled, the Auth section lets you choose which page is used as the initial view.
It has its own navigation stack, so you can e.g. open a registration page with Open page, just like on any other page in your app.
After you've enabled authentication, navigating away from the initial view requires using the Dismiss initial view flow function. You can bypass this requirement in the advanced properties properties for a page by toggling Allow page to be opened without authentication.