Once you complete the "Create a Username and Password" page of your new account, you'll receive an email to verify your email address. After you have verified your email, you can complete your application at any time by logging into the app and tapping Finish Application from the Home page, Portfolio, or anywhere you see the banner within the app.
Alternatively, you can complete the application on the IBKR website. From the home page, click OPEN ACCOUNT and select Finish an Application.