How add tab menu item with external link to article node content
Create custom module  Replace mymdulename with your module name.
implement hook menu to add "Menu local task" which let your menu
item to show along with view, edit ..  
page call back to add the
redirect rule, in below example I am redirecting to google.com, you
can add a check according to node and add redirection rule. 
access callback ensures this menu tabs gets attached to nodes for article
content type.



  /**  * Implements hook_menu().  */

function mymodulename_menu() {
  $items = array();

  $items['node/%node/movies'] = array(
    'title' =&gt; 'Movies',
    'page callback' =&gt; 'mymodulename_redirect',
    'page arguments' =&gt; array('node', 1),
    'access callback' =&gt; 'mymodulename_content_type_check',
    'access arguments' =&gt; array(1),
    'type' =&gt; MENU_LOCAL_TASK,
    'weight' =&gt; 100,
  );

  return $items;
}

/**
* Access callback
*/
function mymodulename_content_type_check($node) {
  if ($node-&gt;type != 'article') {
    return FALSE;
  }
  return TRUE;
}

function mymodulename_redirect($node) {
  dpm($node);
  $url = "http://www.google.com";
  drupal_goto($url, array('external' =&gt; TRUE));
}



Update

to open the external link in new tab use jquery to add '_blank' attribute
in your custom 

jQuery('.tabs a').attr("target","_blank");


Remember to pick unique classes instead of ".tab" to avoid having this behaviour to all tabs.