Accept

 <button
     data-test="trigger"
     onclick="alert('hello!')">Trigger Alert</button>

 import { Target } from '@serenity-js/protractor';
 import { by } from 'protractor';

 class Widget {
     static trigger = Target.the('trigger button')
         .located(by.css('[data-test="trigger"]'));
 }

 import { actorCalled } from '@serenity-js/core';
 import { Accept, BrowseTheWeb, Click, ModalDialog } from '@serenity-js/protractor';
 import { protractor } from 'protractor';

 actorCalled('Nick')
     .whoCan(BrowseTheWeb.using(protractor.browser))
     .attemptsTo(
         Click.on(Widget.trigger),
         Accept.the(ModalDialog.window()),
     );