Add a mailchimp user to a segment

segment.php

<?php

// @link https://mailchimp.com/developer/marketing/api/list-segments/
$segment_id = '123'; // The segment ID
$user_email = '';

wpmem_mc_query_api_lists( 
    'post',
    array( 'segments', $segment_id, 'members' ),
    array( 'email_address' => $user_email )
);