dialog - Post to Friend's Wall using Dialog Feed - How to choose a friend?

Question

We are able to post to any particular friend by providing the 'to' field in dialog feed syntax.

FB.init({appId: "", status: true, cookie: true});

function postToFeed() {
  // calling the API ...
  var obj = {
    method: 'feed',
    redirect_uri: [url],
    link: 'https://developers.facebook.com/docs/reference/dialogs/',
    picture: 'http://fbrell.com/f8.jpg',
    name: 'Facebook Dialogs',
    caption: 'Reference Documentation',
    description: 'Using Dialogs to interact with users.',
    to:[fb id]
  };

  function callback(response) {
    document.getElementById('msg').innerHTML = "Post ID: " + response['post_id'];
  }

  FB.ui(obj, callback);
}

But how do we have a text box which allows the user to choose the friend to whom it has to be posted. Or is there any alternative way?