According to:
https://github.com/brandonseydel/MailChimp.Net/blob/master/README.md You have two options here.
First, you can manually read API key from config:
using mailchimp;
using System.Configuration;
//Read API key from config
var apiKey= ConfigurationManager.AppSettings["APIKey"];
IMailChimpManager manager = new MailChimpManager(apiKey);
Second option is that you can change read key to MailChimpApiKey
, so change
<add key="APIKey" value="XXXX9dsfij4yZXXXXXXXXXX-XXXX" />
to
<add key="MailChimpApiKey" value="XXXX9dsfij4yZXXXXXXXXXX-XXXX" />
In that case you don't need to pass API key:
using mailchimp;
IMailChimpManager manager = new MailChimpManager();