Hello,

The Most Easy way is to add it using a free plugin https://wordpress.org/plugins/insert-headers-and-footers/

If you want to edit the header.php file directly then you need to copy the header.php file from the main theme to the child theme and then try adding the code in it.

You can read the following article for further help:
https://www.wpbeginner.com/plugins/how-to-add-header-and-footer-code-in-wordpress/

Regards.
Xtemos Studios