01-user_custom_field.php

<?php 

function get_user_id_custom_field($field, $value){
  global $wpdb;
  $prepare =  $wpdb->prepare("SELECT user_id FROM $wpdb->usermeta WHERE meta_key = %s AND meta_value = %s ", $field, $value);
  $row = $wpdb->get_row($prepare);
  return isset($row->user_id) ? $row->user_id : 0;
}

02-exemplo.php

<?php 
$code = '21354Adcd';
$user_id = get_user_id_custom_field('voucher', $code);

if($user_id) {
  echo 'Voucher existente';
}