<add key="APIKey.Website" value="MY-KEY" />

ApiConfig.ApiIoC = ioc;
ApiConfig.ApplicationId = appId;
ApiConfig.ApplicationKey = new ApplicationKey(WebConfigurationManager.AppSettings[appKeyConfigKey]);

public static class ApiConfig
{
        private static ApplicationKey _applicationKey;
        public static ApplicationKey ApplicationKey
        {
            get
            {
                if (_applicationKey == null)
                {
                    throw new NotSupportedException($"{nameof(ApiConfig)}.{nameof(ApplicationKey)} not configured.");
                }
                return _applicationKey;
            }
            set
            {
                _applicationKey = value;
            }
        }
}

if (ApiConfig.ApplicationKey.Equals(key)) return null;