Examplary plugin that demonstrates how a developer may hook into the "wpo365/user/user_login" filter in order to customize the WordPress username when WPO365 creates a new user. The filter "wpo365/user/user_login" must be added to WordPress using a custom plugin. Hooking into this filter - for example - in your (child) theme's functions.php will…

wpo365-user-login.php

<?php

/**
 *  Plugin Name: WPO365 | USER-LOGIN
 *  Plugin URI: 
 *  Description: Examplary plugin that demonstrates how a developer may hook into the "wpo365/user/user_login" filter in order to customize the WordPress username when WPO365 creates a new user.
 *  Version: 0.1
 *  Author: marco@wpo365.com
 *  Author URI: https://www.wpo365.com
 */

// Prevent public access to this script
defined('ABSPATH') or die();

/**
 * The filter "wpo365/user/user_login" must be added to WordPress using a custom plugin. 
 * Hooking into this filter - for example - in your (child) theme's functions.php will
 * add the filter after it has been triggered.
 */

add_filter('wpo365/user/user_login', function ($user_login) {
    $_user_login = stristr($user_login, '@', true);
    $_user_login = sanitize_user($_user_login);
    $_user_login = sprintf('%s-%s', time(), $_user_login);
    return $_user_login;
}, 10, 1);