Put this in your page somewhere:

<script src="https://captcheck.extinctionrebellion.uk/captcheck.min.js"></script>

Put this in your form where you want the CAPTCHA:

<div class="captcheck_container"></div>

Put this in your server-side form validation (PHP example):

$url = 'https://captcheck.extinctionrebellion.uk/api.php';
$data = [
    'session_id' => $_POST['captcheck_session_code'],
    'answer_id' => $_POST['captcheck_selected_answer'],
    'action' => "verify"
];
$options = [
    'http' => [
        'header' => "Content-type: application/x-www-form-urlencoded\r\n",
        'method' => 'POST',
        'content' => http_build_query($data)
    ]
];
$context = stream_context_create($options);
$result = file_get_contents($url, false, $context);
$resp = json_decode($result, TRUE);
if (!$resp['result']) {
    // Replace with error-handling code
    exit("CAPTCHA did not verify:" . $resp['msg']);
} else {
    // The CAPTCHA is valid.
    exit("CAPTCHA verified!");
}

If you have a strict Content Security Policy, change your div to this:

<div class="captcheck_container" data-stylenonce="your nonce here"></div>