Adding this to your theme will redirect any requests for wp-signup.php (triggered upon visiting a multisite network address that doesn't exist) to the network home URL along with some GA parameters

header-wp-signup.php

<?php
/**
 * @package Header for WP Signup
 * @author  James Morrison 
 * @link    https://gist.github.com/jamesmorrison/5f8a3108b2a773cd07db430b73992e9d/edit
 *
 * This file is called by the WP core /wp-signup.php file; we can force a redirect from here
 *
 * Whilst you can do this from the NOBLOGREDIRECT constant, the advantage of using this header
 * file is that you can capture the path that triggered the call to signup; as seen in the example
 * below, this uses Google Analytics parameters to help track where the requests came from without
 * the need to setup infrastructure level redirects.
 *
 * Additionally, NOBLOGREDIRECT has a nasty habit of messing about with 404 pages - in some cases
 * it treats these as an invalid network site and triggers the redirect instead of showing the 404 page
 *
 **/

wp_redirect(
	esc_url_raw(
		add_query_arg( [
			'utm_source'   => isset( $_GET[ 'new' ] ) ? esc_attr( $_GET[ 'new' ] ) : '',
			'utm_medium'   => 'referral',
			'utm_campaign' => 'no-blog-redirect' 
		], network_home_url() )
	),
301 );

exit();