Code-Snippets-Functions/Execute a function on a child site/WooCommerce/add-custom-field-checkout-block.txt

// Add a custom checkout field (checkout blocks)
add_action(
    'woocommerce_init',
    function() {
        woocommerce_register_additional_checkout_field( array(
            'id'          => 'namespace/newsletter-opt-in',
            'label'       => 'Subscribe to newsletter?',
            'location'    => 'order',
            'type'        => 'checkbox',
        ) );
    }
);

// Save the custom checkout field value (when the checkbox ix checked)
add_action(
    'woocommerce_set_additional_field_value',
    function ( $key, $value, $group, $wc_object ) {
        if ( 'namespace/newsletter-opt-in' === $key ) {
            $wc_object->update_meta_data( 'newsletter_opt_in', $value );
        }
    },
    10,
    4
);