discourse/app/assets/javascripts/admin/addon/routes/admin-customize-email-style-index.js

import Route from "@ember/routing/route";
import { inject as service } from "@ember/service";

export default class AdminCustomizeEmailStyleIndexRoute extends Route {
  @service router;

  beforeModel() {
    this.router.replaceWith("adminCustomizeEmailStyle.edit", "html");
  }
}