Extended Hubbub Pro's Save This email to include a link from an ACF field

hubbub_filter_acf.php

<?php

function hubbub_save_this_filter_add_book_button( $email_message, $post_id, $is_shortcode ) {

  if (class_exists('ACF')) { // Check to make sure Advanced Custom Fields is active
    $bookURL = get_field( $selector, $post_id );
    if ( ! $bookURL ) $bookURL = '';

    $email_message = str_replace( '{BOOK_BUTTON}', '<a href="' . $bookURL .'">Download Book</a>', $email_message );
  }
  
  return $email_message;
}
add_filter( 'hubbub_save_this_filter_email_message', 'hubbub_save_this_filter_add_book_button' 10, 3 );