ModernBranding.ts

export default class HelloWorldApplicationCustomizer extends BaseApplicationCustomizer {

  private _externalJsUrl: string = "https://{yourtenant}.sharepoint.com/CDN/CustomStyling.css";
  private _externalJsUrl2: string = "https://{yourtenant}.sharepoint.com/CDN/CustomScript.js";
  private _externalJsUrl3: string = "https://{yourtenant}.sharepoint.com/CDN/jquery-3.3.1.min.js";

  public onInit(): Promise {
    Log.info(LOG_SOURCE, `Initialized ${strings.Title}`);

    // Added to handle possible changes on the existence of placeholders.
    this.context.placeholderProvider.changedEvent.add(this, this._renderPlaceHolders);

    // Call render method for generating the HTML elements.
    this._renderPlaceHolders();
    return Promise.resolve();
  }

  private _renderPlaceHolders(): void {
    let scriptTag1: HTMLLinkElement = document.createElement("link");
    scriptTag1.rel = "stylesheet";
    scriptTag1.href = this._externalJsUrl;
    scriptTag1.type = "text/css";
    document.getElementsByTagName("head")[0].appendChild(scriptTag1);

    let scriptTag2: HTMLScriptElement = document.createElement("script");
    scriptTag2.src = this._externalJsUrl2;
    scriptTag2.type = "text/javascript";
    document.getElementsByTagName("head")[0].appendChild(scriptTag2);

    let scriptTag3: HTMLScriptElement = document.createElement("script");
    scriptTag3.src = this._externalJsUrl3;
    scriptTag3.type = "text/javascript";
    document.getElementsByTagName("head")[0].appendChild(scriptTag3);

   
  }
}