Noindex posts only for google without plugins

Hi. I’m trying to allow noindexing posts and pages (for Google) through a custom field using this code:

$noindex = get_post_meta($post->ID, 'noindex-page', true);
if ($noindex) {
    echo '<meta name="googlebot" content="noindex,follow" />';
}

The code needs to be in the header, so I’m setting the location in WPCode to “Site Wide header” and activated it.
I’m creating the “noindex-page” custom field in the WordPress editor and putting a value in it for the desired post, but nothing occurs when checking the source code of the post.

I’m wondering if i shouldn’t maybe create the custom field by code; could you please advise me on how to solve this issue?

Thank you very much.

PS: I’m using:
– WordPress: 6.4.3.
– GeneratePress Theme: 3.4.0.
– WPCode Lite: 2.1.10.