How do I specify ECMASCRIPT6 as a --language_in parameter in the JavaScriptMinifyWorkAction task?

I am having a problem converting my application from sbt to gradle, the error is like this

```
A failure occurred while executing org.gradle.playframework.tasks.internal.JavaScriptMinifyWorkAction
Minification failed with the following errors:
ES6_FEATURE. this language feature is only supported in es6 mode: const declarations. Use --language_in=ECMASCRIPT6 or ECMASCRIPT6_STRICT to enable ES6 features. at /app/app/assets/javascripts/characters.js line 69 : 18
```

so I understand that I need to add this feature, but I don't know how to do this in my configuration and I can't find any documentation on how to configure how the task will work

does anybody here know how?