With the flutter CLI we can run:

$ flutter attach --device-id=flutter-tester --debug-uri=http://[....] --target=lib/main_desktop.dart 

The target flag is used to select The main entry-point file of the application, as run on the device, if omitted, it default to "lib/main.dart").

Currently the extension doesn't support custom entry-point, ref, "lib/main.dart" is always used.