Where do I go to add a tag before the closing body tag?

Question

Seems like you need to put this in your header within your <head> tags before your opening <body> tag.

Put the following into your theme’s function.php file:

function wpse_285611_pinterest_rich_pins() {
    //change 123 to the ID of the post OR
    //change 123 to the slug or title of the post
    if ( is_single(123) ) {
        echo '<meta name="pinterest-rich-pin" content="false" />';
    }
}

add_action('wp_head', 'wpse_285611_pinterest_rich_pins', PHP_INT_MAX);

See: https://developer.wordpress.org/reference/functions/is_single/ for information on how to use the is_single conditional template tag if you want to restrict this to one post only.