To setup your GetResponse integration with Ninja Shop go to Ninja Shop > Features and click on the gear for GetResponse

To obtain your GetResponse API key, login to your GetResponse account and go to Profile > Integrations & API > API

Once you have added your API key, you should be able to choose the appropriate list

You can also change the text of the label for the GetResponse opt-in and choose if the GetResponse Opt-in is checked by default.