Insert this code into the functions.php file within your WordPress child-theme to inject any custom code into the HEADER of your WordPress website.

wordpress-header-hook.php

<?php

// Inserting a Script into WordPress Header
// https://endurtech.com/insert-script-into-wordpress-header/

// wp_head action hook to inject a script
add_action( 'wp_head', 'custom_header_scripts' );
function custom_header_scripts()
{
	echo '<script>alert( "This is triggered from within the header of your website." );</script>';
}

// Alternatively, you can stop and restart PHP processing to inject your code
// which helps bypass potential issues with apostrophes or quotations
add_action( 'wp_head', 'custom_header_scripts' );
function custom_header_scripts()
{
	?>
<script>alert( "This is triggered from within the header of your website." );</script>
	<?php
}

?>