Go to settings--> Applications --> Pick your app ( Mailchimp, MailerLite, Moosend, Getresponse)--> click connect, Then add your API key In the box you got from your the 3rd party app. Finally, save and verify. That's simple!
ActiveCampaign bit different, But still need API key and their host API name as you see in the screenshot below. You may contact ActiveCampaign support if you can not find the host API name...