var instrumentationKey = ConfigurationManager.AppSettings["ApplicationInsightsInstrumentationKey"];

if (string.IsNullOrWhiteSpace(instrumentationKey))
{
    throw new ConfigurationErrorsException("Missing app setting 'ApplicationInsightsInstrumentationKey' used for Application Insights");
}

TelemetryConfiguration.Active.InstrumentationKey = instrumentationKey;

new TelemetryClient().TrackEvent("Application started");