Where do i find my extension id?

API
9 
function create_app_access_token() {
    // Twitch-Anmeldedaten
    $clientId = 'XXX';
    $clientSecret = 'XXX';

    $tokenURL = 'https://id.twitch.tv/oauth2/token';
    $params = array(
        'client_id' => $clientId,
        'client_secret' => $clientSecret,
        'grant_type' => 'client_credentials'
    );

    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $tokenURL);
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($params));
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

    $response = curl_exec($ch);
    curl_close($ch);

    $responseData = json_decode($response, true);
    if (isset($responseData['access_token'])) {
        $accessToken = $responseData['access_token'];
        return $accessToken;
    } else {
        return false;
    }
}

function get_extension_stats() {
    $accessToken = create_app_access_token();
    $extensionID = 'XXX';

    $apiURL = 'https://api.twitch.tv/helix/extensions/transactions';
    $params = array(
        'extension_id' => $extensionID
    );

    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $apiURL . '?' . http_build_query($params));
    curl_setopt($ch, CURLOPT_HTTPHEADER, array(
        'Client-ID: XXX',
        'Authorization: Bearer ' . $accessToken,
    ));
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    $response = curl_exec($ch);
    curl_close($ch);

    // Antwort verarbeiten
    $responseData = json_decode($response, true);
    if (isset($responseData['data'])) {
        $transactions = $responseData['data'];
        var_dump($transactions);
    } else {
        var_dump($responseData);
    }
}