title: Retrofit: Add a String List parameter to Multipart Request
body: <p>I'm trying to add a String list parameter to a multi-part request.</p>
  
  <p>Using Apache Http, I set the parameter like this:</p>
  
  <pre><code>MultipartEntity mpEntity = new MultipartEntity(HttpMultipartMode.BROWSER_COMPATIBLE);
  HttpPost httpPost = new HttpPost(url);
  httpPost.setEntity(mpEntity);
  
  for (User member : members) {
      mpEntity.addPart("user_ids[]", new StringBody(member.id.toString()));
  }
  </code></pre>
  
  <p>How do I do this on Retrofit?</p>