Hello,

The below code will help you to add this meta to the head tag, please paste it to the functions.php file in your child theme.

add_action(
	'wp_head',
	function() {
		echo '<meta name="p:domain_verify" content="add551991282887361728df9bf830974"/>';
	}
);

You can also reach out to us if you have any questions.

Kind Regards