How to call event handler onload for Angular application?

I want to open a window containing user authorization form details. I want to do this by checking window location, to do so the event handler not call on every event.  I am not able to overwrite event handlers.

Why the event is not triggering? what could be the alternative way to intercept the Authorization form result?

My code:

import { Inject, Injectable } from '@angular/core';

// ... other imports will be go here. //

@Injectable({
 providedIn: 'root'
})
export class UserWindowService {

 constructor(@Inject(WINDOW) public window: Window) {} // here WINDOW is imported from InjectionToken<Window>

 openUserWindow(): void {
 let windowObj = this.window.open('//localhost:4200/verify', '_blank');
 let evntHandler= function() {
 console.log(windowObj!.location);
 };
 //Below order of events should execute one by one, one after one.
 windowObj!.onload = evntHandler;
 windowObj!.addEventListener('load', evntHandler);
 windowObj!.addEventListener('onload', evntHandler);
 windowObj!.document.onload = evntHandler;
 windowObj!.onhashchange = evntHandler;
 windowObj!.addEventListener('hashchange', evntHandler, false);
 }
}