$expertSender = new ExpertSender('https://api.esv2.com/', $apiKey);

$customProperty = new Property(1775, ExpertSenderEnum::TYPE_STRING, 'female');

$request = new AddUserToList();
$request
    ->setEmail('my@email.com')
    ->setListId(1000)
    ->setFirstName('my name');
    ->addProperty($customProperty)
    ->freeze();

$result = $expertSender->addUserToList($request);

if ($result->isOk()) {
    ...
} else {
    ...
}