After activating the user, redirects to the reset user password WP default page.

wpmudev-forminator-reset-user-password.php

<?php
/**
 * Plugin Name: [Forminator] Reset User Password
 * Plugin URI: https://wpmudev.com
 * Description: After activating the user, redirects to the reset user password WP default page.
 * Author: Glauber Silva @ WPMUDEV
 * Author URI: https://wpmudev.com
 * Jira Task: SLS-3313
 * License: GPLv2 or later
 *
 * @package WPMUDEV_Forminator_Reset_User_Password
 */
defined( 'ABSPATH' ) || exit;

function wpmudev_forminator_reset_user_password( $user_id, $signup_meta ) {    

    if ( ! is_admin() ) {
        $user = get_userdata( $user_id );
        $key  = get_password_reset_key( $user );
        $url  = esc_url_raw( untrailingslashit( wp_login_url() ) . "?action=rp&key=$key&login=" . rawurlencode( $user->user_login ) );
        wp_redirect( $url );
        exit();    
    }
}
add_action( 'forminator_activate_user', 'wpmudev_forminator_reset_user_password', 10, 2);