From https://semperplugins.com/support/general-subscribe2-html-support-forum/managing-licenses-for-multisite/#p1215

subscribe2_html_license_propagater.php

<?php
/*
Plugin Name: Subscribe2 HTML License Propagater
Plugin URI: http://semperplugins.com/plugins/subscribe2-html/
Description: Propagates a Subscribe2 HTML License Key over a Multisite Install
Version: 1.0a
Author: Matthew Robinson
Author URI: http://semperplugins.com/plugins/subscribe2-html/
Licence: GPL3
Donate link: https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&amp;hosted_button_id=2387904
*/
define( 'S2LICENCEKEY', '' );

register_activation_hook(__FILE__, 'propagate_s2_init');
add_action('propagate_s2_cron', 'propagate_s2_license');

function propagate_s2_init() {
$propagate_scheduled = wp_next_scheduled('propagate_s2_cron');
// add the cron event on upgrade if it does not exist
if ( $propagate_scheduled === false ) {
wp_schedule_event(current_time('timestamp'), 'every12hours', 'propagate_s2_cron');
}
} // end propagate_s2_init()

function propagate_s2_license() {
global $wpdb, $mysubscribe2;

if ( !$mysubscribe2->s2_mu ) { return; }

$blogs = $wpdb->get_col("SELECT blog_id FROM {$wpdb->blogs}");

if ( !function_exists('is_plugin_active') ) {
include_once( ABSPATH . 'wp-admin/includes/plugin.php' );
}

foreach ( $blogs as $blog ) {
switch_to_blog($blog);

if ( is_plugin_active('subscribe2_html/subscribe2.php') ) {
$subscribe2_options = get_option('subscribe2_options');
}
$subscribe2_options['s2_license_key'] = S2LICENCEKEY;
update_option('subscribe2_options', $subscribe2_options);

restore_current_blog();
}
} // end propagate_s2_license()
?>