Returns a short-lived portal link for the given target. Unlike Requirement.resolutionMethods[].url or the deprecated Onboarding.portalUrl, this link expires shortly after being issued — do not request it ahead of time and store it. Call this endpoint at the moment your user needs to be redirected (e.g. when they click a link on your platform), then redirect them to the returned url right away. See Getting a fresh portal link.