GetResponse is an application that makes email marketing easier. This action allows you to create a connection between your form and the GetResponse account. The data from the form can be used through the GetResponse functionalities.
As for every third-party application of that type, you will need to get the API key to create a connection. When you obtain it, paste it to the API Key bar and press the “Validate API Key” button.
You will be able to read about the process of getting the API key in more detail here. However, be aware that this page is available only to logged-in users.