Authorize
In order to connect your iContact account with your Mautic, you'll have to create a iContact app. Follow the tutorial how to create your iContact APP.
When you have your app created, you should be able to see this screen:
Fill it the right credentials to Mautic - iContact integration:
App ID = the Application ID you've created
App username = the username / email you log into your iContact account. (Not the App name) App password = The password you've chosen when approving the app.