buddypress customize user activation email to admin

functions.php

<?php
add_filter( 'bp_core_sent_user_validation_email', 'custom_buddypress_activation_message', 1, 4 );
function custom_buddypress_activation_message( $subject, $message, $user_id, $meta ) {

    $user_meta = get_user_meta($user_id, 'activation_key', OBJECT);

    $subject = '[SIEBA] Activate Users';
    $message .= 'Activate User With';
    $message .= 'Username: '.$_POST['signup_username'].'<br>';
    $message .= 'Email: '.$_POST['signup_email'].'<br>';
    $message .= 'To complete the activation of new account please click the following link: </br>';
    $message .= '<a href="'.esc_url( home_url( "/activate/" ) ).$user_meta.'">';
    $message .= esc_url( home_url( "/activate/" ) ).$user_meta;
    $message .= '</a></br>';
    $message .= '<a href="'.esc_url( home_url( "/activate/" ) ).$user_meta.'">By clicking here...</a>';

    wp_mail( get_option('admin_email'), $subject, $message);

}
?>