Angular

/** Leave for Authentica by POSTing the hand-off token. Nothing goes in the URL. */
export function postToVerify(
  verifyUrl: string,
  handoff: string,
  extras: Record<string, string | undefined> = {},
): void {
  const f = document.createElement('form');
  f.method = 'POST';
  f.action = verifyUrl;                 // bare endpoint, no query string
  f.style.display = 'none';

  const add = (name: string, value: string) => {
    const i = document.createElement('input');
    i.type = 'hidden';
    i.name = name;
    i.value = value;
    f.appendChild(i);
  };

  add('hx', handoff);                   // the handoffField from /session
  Object.entries(extras).forEach(([k, v]) => v && add(k, v));

  document.body.appendChild(f);
  f.submit();
}

import { postToVerify } from '../core/verify-handoff';

async startVerify() {
  const r = await fetch("/verify/start", {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    credentials: "include",
    body: JSON.stringify({
      action: "payment",              // your own value — login, phone-change, approval, …
      params: { paymentId: 8842 },    // whatever that action needs
      destination: "0501234567",
    }),
  });
  const data = await r.json();
  if (!data.verifyUrl || !data.handoff) throw new Error("no handoff");
  postToVerify(data.verifyUrl, data.handoff);   // optional: { lang: 'ar' }
}

ngOnInit(): void {
  // Ask YOUR backend — it is the one that called GET /session/{id}.
  this.api.verifyResult(window.location.search).subscribe({
    next: (res) => this.router.navigateByUrl(res.nextRoute ?? '/'),
    error: () => { this.error = 'verification_failed'; },
  });
}