Redirect WordPress Multisite User Upon Activation

redirect-wp-multisite-activation.php

<?php 
// add do_action('my_do_active_user_message_hook'); where you want the welcome message to display after redirect

add_action( 'activate_header', 'check_activation_key_redirect_to_page' );
/**
 * Check the wp-activate key and redirect the user to the apply page
 * based on http://www.vanbodevelops.com/tutorials/how-to-skip-the-activation-page-and-send-the-user-straight-to-the-home-page-for-wordpress-multisite
 */
function check_activation_key_redirect_to_page() {
    // We check if the key is not empty
    if ( ! empty($_GET['key']) || ! empty($_POST['key']) ) {

        $key = !empty($_GET['key']) ? $_GET['key'] : $_POST['key'];

        // Activates the user and send user/pass in an email
        $result = wpmu_activate_signup($key);

        if ( ! is_wp_error($result) ) {
            extract($result);
            $user = get_userdata( (int) $user_id);

            // Save the user object to the session
            setcookie('my_active_user_variable', json_encode($user));

            // Redirect to the network home url
            wp_redirect( network_site_url() );
            exit;
        }
    }
}
add_action('my_do_active_user_message_hook', 'check_header_for_active_user');
/**
 * Check for the set activated user and echo a message on the home page
 */
function check_header_for_active_user() {
    if ( isset($_COOKIE['my_active_user_variable']) && ! empty( $_COOKIE['my_active_user_variable'] ) ) {
        $user = json_decode($_COOKIE['my_active_user_variable']);

        echo '<div class="app-msg">'. sprintf( __( 'Dear %s. Your account is now active!' ), $user->user_login ).'</div>';

        // Unset the session variable since we don't needed anymore
        unset($_COOKIE['my_active_user_variable']);
    }
}

how to redirect to subdomain homepage instead of main site ? any idea psl

To redirect to the current site instead of the main site, change wp_redirect( network_site_url() ); to wp_redirect( site_url() );