Add to mu-plugins folder, auto add every new user to a sendy list. Works with default WordPress and Woocommerce registrations.

sendy.php

<?php
add_action( 'user_register', 'add_user_to_sendy_list' );

function add_user_to_sendy_list( $user_id ) {
  $list = 'SENDY_LIST_ID';
  $url  = 'http://SENDY_INSTALL_URL/subscribe';
  $user = get_userdata( $user_id );
  
  $email = $user->data->user_email;
  $name  = $user->data->user_nicename;
  
  $args = array(
    'body' => array(
      'email' => $email,
      'name'  => $name,
      'boolean' => true,
      'list'    => $list
    )
  );
  
  $result = wp_remote_post( $url, $args );
}