// omitting other imports for brevity...
import {AppInsights} from "applicationinsights-js";

export default class AppInsightsCustomizerApplicationCustomizer
  extends BaseApplicationCustomizer<IAppInsightsCustomizerApplicationCustomizerProperties> {

  @override
  public onInit(): Promise<void> {
    let appInsightsKey: string = "/* update with YOUR App Insights key: */";
    AppInsights.downloadAndSetup({ instrumentationKey: appInsightsKey });

    // simple usage - all params will be derived..
    AppInsights.trackPageView();

    return Promise.resolve<void>();
  }
}