Where to add custom proguard rule

cordova
1

With cordova-plugin-proguard 2.0.0 installed and checked official doc specifying:
copy proguard-custom.txt to ${androidPlatformDirectory}/assets/www/proguard-custom.txt

Question:
Where to add custom proguard rule?

Observation:

  1. Tried manually create file platforms/android/platform_www/proguard-custom.txt
    Result: effective but if pull source as clean project, cannot build as
    The platform "android" does not appear to be a valid cordova platform.

  2. Tried manually create directory platforms/android/assets/www/proguard-custom.txt
    Result: not effective

The main confuse is that official document does not specify raw source of proguard custom rule of user should edit.

thanks