After installing launch_at_login by adding to the pubspec.yaml and running flutter pub get -v
It will return the path of the launch_at_login lib (something like: ~/.pub-cache/hosted/pub.dartlang.org/launch_at_login-0.0.3/lib/) in which there will be an assets folder with a file called copy-login-helper.sh.
Add to your macos Xcode project in the Build Phases -> Run Script:
/bin/sh ~/.pub-cache/hosted/pub.dartlang.org/launch_at_login-0.0.3/lib/assets/copy-login-helper.sh
You can find the .pub-cache directory by looking in your projects .flutter-plugins-dependencies file.
this builds the helper app to launch your MacOS app on login.
Then you can run:
$ flutter run -d macos