For lightbox videos:

function change_magnific_popup_iframe_setting(){
?>
<script>
jQuery(window).load(function(){
	jQuery('a[href*="youtube.com/watch"]').magnificPopup({
	   type: 'iframe',
	   iframe: {
	     patterns: {
	       youtube: {
	       	index: 'youtube.com', 
	       	id: 'v=', 
	       	src: '//www.youtube.com/embed/%id%?rel=0&autoplay=1'
	       }
	     }
	   }   
	});      
});
</script>
<?php
}
add_action("wp_footer", "change_magnific_popup_iframe_setting”);