How to add code to head (via functions.php)

Paste the following code in your theme’s functions.php:

/* Describe what the code snippet does so you can remember later on */
add_action('wp_head', 'your_function_name');
function your_function_name(){
?>
PASTE HEADER CODE HERE
<?php
};

Add description (comment above the code snippet) and change the function’s name (two locations). Then add the desired code replacing “PASTE HEADER CODE HERE”.

Source: https://kinsta.com/knowledgebase/add-code-wordpress-header-footer/