How to use global merge tags snippet?

I am trying to implement the snippet you posted here:

https://docs.bracketspace.com/notification/developer/triggers/adding-merge-tags-to-existing-triggers#global-merge-tags
first in my theme’s functions.php file, then as a mu-plugin, but all I get when I reload the Notifications admin pages is:

Fatal error: Uncaught Error: Class ‘BracketSpace\Notification\Register’ not found in /home/xxxxxxx/public_html/xxxxxxxxxx.org/wp-content/mu-plugins/notifications-global-mergetag.php:13

What am I missing? Here is a generic version of my plugin. I tried it with and without the namespace declaration.

<?php
/**
* Plugin Name: Global Merge Tag for Notifications
* Description: Create global merge tag for Site Name
* Author: BracketSpace
*/

namespace MG2022notifications;

use BracketSpace\Notification\Defaults\MergeTag\StringTag;
use BracketSpace\Notification\Register;

Register::global_merge_tag( new StringTag( [
	'slug'     => 'site_name',
	'name'     => 'Site Name',
	'resolver' => function( $trigger ) {
		return get_option( 'blogname', 'Website Name' );
	},
] ) );

Thanks in advance.