Note:
  On versions of Flutter where flutter build ipa --export-method is unavailable,
  open build/ios/archive/MyApp.xcarchive and follow the instructions below
  to validate and distribute the app from Xcode.

Upload the app bundle to App Store Connect

Once the app bundle is created, upload it to
App Store Connect by either:

Install and open the Apple Transport macOS app.
Drag and drop the build/ios/ipa/*.ipa app bundle into the app.

Or upload the app bundle from the command line by running:
    
xcrun altool --upload-app --type ios -f build/ios/ipa/*.ipa --apiKey your_api_key --apiIssuer your_issuer_id
    
    Run man altool for details about how to authenticate with the App Store Connect API key.

Or open build/ios/archive/MyApp.xcarchive in Xcode.