In Android Studio's tab bar, click on Tools and then Flutter and then Open Android module in Android Studio if you don't find Android module in Android Studio, Open (open for editing in android studio)
Open the project in a new window
Having opened the project, click on Build and then Generate Signed Bundle / APK ...
Select Create new.. option to generate new Signed key (When you release your app First Time). But you can choose existing if you have already generated a signed key