Create custom meta tag with the metatag module
You can either use Custom Meta module or roll your own module:

From Custom Meta project page:


  The module allows you to define and manage custom meta tags. Simply
  select the Meta attribute (property, name, http-equiv) and set the
  value and content value in the format and the module will add your
  meta tags to all non-admin content for your site.


An example of writing your own module:

my_metatag.info  

name = My Metatags  
description = Provides my custom Metatags.  
core = 7.x  
version = 7.x-1.x  
dependencies[] = metatag  
files[] = my_metatags.metatag.inc


my_metatag.module

&lt;?php

/**
 * Implements hook_ctools_plugin_api().
 */
function my_metatags_ctools_plugin_api($owner, $api) {
  if ($owner == 'metatag' &amp;&amp; $api == 'metatag') {
    return array('version' =&gt; 1);
  }
}


my_metatag.metatag.inc

&lt;?php
//
// Implements hook_metatag_info().
//
function my_metatags_metatag_info() {
  $info['groups']['my_metatags'] = array(
    'label' =&gt; t('My Custom Metatags'),
  );

  $info['tags']['my_custom_metatag'] = array(
    'label' =&gt; t('My Custom Meta Tag'),
    'description' =&gt; t('This is a custom meta tag'),
    'class' =&gt; 'DrupalTextMetaTag',
    'group' =&gt; 'my_metatags',
  );

  return $info;
}

//
// Implements hook_metatag_config_default_alter().
//
function my_metatags_metatag_config_default_alter(array &amp;$configs) {
  foreach ($configs as &amp;$config) {
    switch ($config-&gt;instance) {
      case 'global':
        $config-&gt;config += array();
        break;

      case 'global:frontpage':
        $config-&gt;config += array();
        break;

      case 'node':
        $config-&gt;config += array(
          'my_custom_metatag' =&gt; array('value' =&gt; 'This is a default value.'),
        );
        break;

      case 'taxonomy_term':
        $config-&gt;config += array();
        break;

      case 'user':
        $config-&gt;config += array( );
        break;
    }
  }
}


Hope that helps.