add external js reference on every page using spfx

 //define a property
 private _externalJsUrl: string = "https://extrenalSP.azurewebsites.net/scripts/SPFxGlobalScript.js";

 //add below code snippet on onInit() method
  @override
  public onInit(): Promise<void> {
    console.log(`IHideSiteContentApplicationCustomizerProperties.onInit(): Entered.`);
    let scriptTag: HTMLScriptElement = document.createElement("script");
    scriptTag.src = this._externalJsUrl;
    scriptTag.type = "text/javascript";
    document.getElementsByTagName("head")[0].appendChild(scriptTag);
    console.log(`IHideSiteContentApplicationCustomizerProperties.onInit(): Added script link.`);
    console.log(`IHideSiteContentApplicationCustomizerProperties.onInit(): Leaving.`);
    return Promise.resolve();
}