So in the Settings / Embed Facebook Ads Tracking Code, simply put the pixel in first, then paste the event in below it, like so:
<!-- 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', '2591031754288929');
fbq('track', 'PageView');
</script>
<noscript><img height="1" width="1" style="display:none"
src="https://www.facebook.com/tr?id=2591031754288929&ev=PageView&noscript=1"
/></noscript>
<!-- End Facebook Pixel Code -->
<script>
fbq('track', 'CompleteRegistration', {
value: 1,
});
</script>
Note that for "track event on page load", you only need the head script (the yellow above), you don't need the stuff that comes after the <body> tag