In the Modeler, go to Project > Security > Demo Users. There you can create and view demo users that you can use to log onto your app when it is running in the Sandbox. Passwords for demo users are automatically created, but you can copy them to clipboard and then use them to log in.
This allows you to log in as an Administrator for your app if the Demo User has the correct role assigned.
You can use the method that I mentioned in this post. This will create a user account at start up so you can log into the application. Another option is to turn security off, deploy, create an admin account, turn security back on, and redeploy.
Hope this helps!