To use Auto Sign In your school must use an authenticator like "Sign In with Microsoft" or similar. This will create a URL in your web history that can be used to automatically sign in using PlannerPlus.
To get this link you must first sign into the app you want Auto Sign-In to be set up on.
Simply, Use the "Sign In with Microsoft" button, then complete anything they require you to sign in.
Once this is done, use your web browser's history function and search for "Working...", this may be different depending on your email provider (Not using outlook)
Now copy this "Working" link into PlannerPlus, and press "Save Data" and make sure Active is ticked.