This hook worked and was able to be verified immediately for my Facebook Pixel.
Create a file. ( I named it FBPixel.php ) in: /includes/hooks/FBPixel.php
Add the following code and replace 123456 for your Pixel Id. or copy your pixel code in between the "return <<<HTML and HTML;" tags.
Save and test your pixel. Mine was immediately verified. You can also view the source code of your page and you can see the pixel code below there.
Hope this helps.
JC
<?php
/**
* Facebook Pixel Code
* @author JC
*/
add_hook('ClientAreaHeadOutput', 1, function($vars) {
return <<<HTML
<!-- Facebook Pixel Code -->
<script>
!function(f,b,e,v,n,t,s)
{if(f.fbq)return;n=f.fbq=function(){n.callMethod?
n.callMethod.apply(n,arguments):n.queue.push(arguments)};
if(!f._fbq)f._fbq=n;n.push=n;n.loaded=!0;n.version='2.0';
n.queue=[];t=b.createElement(e);t.async=!0;
t.src=v;s=b.getElementsByTagName(e)[0];
s.parentNode.insertBefore(t,s)}(window, document,'script',
'https://connect.facebook.net/en_US/fbevents.js');
fbq('init', '123456'); //Your pixel ID here.
fbq('track', 'PageView');
</script>
<noscript><img height="1" width="1" style="display:none"
src="https://www.facebook.com/tr?id=123456&ev=PageView&noscript=1"
/></noscript>
<!-- End Facebook Pixel Code -->
HTML;
});