title: Redirect user to external URL in controller action
body: <p>I am trying to add online payment to my application and it requires me to post a form to their website like so.  It has a few required hidden inputs (I'm just showing one of them for the transaction total).</p>
  
  <pre><code>&lt;form action="https://thirdpartysite.com/clientpostback" method="POST"&gt;
      ...
      &lt;input type="hidden" name="TransTotal" id="TransTotal" value="135.00" /&gt;
      ...
  &lt;/form&gt;
  </code></pre>
  
  <p>Instead of posting to their website from the view, I want to post to a Controller in my application so I can update their application saying they agreed to the terms and conditions.</p>
  
  <pre><code>[HttpPost]
  public ActionResult Review(ApplicationReviewViewModel vm)
  {
      if (ModelState.IsValid())
      {
          _appBusiness.Update(vm);
  
          // Now I need to post to the third party website and pass in the posted values
          return RedirectToAction("https://thirdpartysite.com/clientpostback");
      }
  }
  </code></pre>
  
  <p>How would I redirect users to the third party site from my controller action?  Is that possible?</p>