Facebook messenger on web page

I'm repeatedly trying to integrate Facebook chat on a client's website without effectiveness. If I try with the fanpage of an example on the subject, it works correctly.

<div id="fb-root"></div>
<script>

  window.fbAsyncInit = function() {
    FB.init({
      appId      : 'XXXXXXXXXX',
      xfbml      : true,
      version    : 'v2.8'
    });
  };

  (function(d, s, id){
    var js, fjs = d.getElementsByTagName(s)[0];
    if (d.getElementById(id)) {return;}
    js = d.createElement(s); js.id = id;
    js.src = "//connect.facebook.net/es_LA/sdk.js";
    fjs.parentNode.insertBefore(js, fjs);
  }(document, 'script', 'facebook-jssdk'));

</script>

<div class="fb-page" 
     data-href="https://www.facebook.com/XXXXXXXXXX/" 
     data-tabs="messages" 
     data-small-header="true" 
     data-adapt-container-width="true" 
     data-hide-cover="true" 
     data-show-facepile="true">
  <div class="fb-xfbml-parse-ignore">
    <blockquote>
      <a href="https://m.me/XXXXXXXXXX" class="fb-msg-btn" target="_blank">¡Consultar precio o comenzar compra!</a>
    </blockquote>
  </div>
</div>

I tried the name of the FP, with its ID, using the ID of a new generated APP for this (bare, without any configuration).

Maybe the APP needs some configuration linked to Messenger, or link it in a certain way with the FanPage in question.