Я нашел ответ на свой вопрос:
Чтобы добавить подписку для пользователя, вы должны сначала добавить платеж, а затем подписку.
$customer = $mollie->customers->create([
"name" => $fullName,
"email" => $email,
]);
$payment = $customer->createPayment([
"amount" => [
"currency" => $currency,
"value" => $amount,
],
"description" => $description,
"redirectUrl" => $siteUrl,
"webhookUrl" => $webhook,
"metadata" => [
"order_id" => $orderId,
],
"sequenceType" => \Mollie\Api\Types\SequenceType::SEQUENCETYPE_FIRST,
]);
$customer->createSubscription([
"amount" => [
"currency" => $currency,
"value" => $amount,
],
"times" => $recurringLimit,
"interval" => $interval,
"description" => $description,
"webhookUrl" => $webhook,
"method" => NULL,
]);