$response = Http::withOptions(['stream' => true])->post($url, $payload);
$body = $response->toPsrResponse()->getBody();

while (! $body->eof()) {
    $line = trim(\GuzzleHttp\Psr7\Utils::readLine($body));
    // ...
}

$response = Http::stream()->post($url, $payload);

foreach ($response->lines() as $line) {
    // ...
}