Getting “Sorry, please enter the key” on PHP cUrl request

I am following the API Redeem Microsoft call from this website: https://getcid.info/developers]

Here is my PHP code for the cURL request:

$url = 'https://getcid.info/api-key-digital';
 
$curl = curl_init();

$fields = [
    'key' => '1231232-1232123-1232123-1231232-5762636-1232123-1232123-1232123-1232123'
];

$fields_string = http_build_query($fields);
 
curl_setopt($curl, CURLOPT_URL, $url);
curl_setopt($curl, CURLOPT_POST, TRUE);
curl_setopt($curl, CURLOPT_POSTFIELDS, $fields_string);
 
$data = curl_exec($curl);

echo '<pre>';
     var_dump( $data );
echo '</pre>';
 
curl_close($curl);

But I am getting an error message;

{“status”:”error”,”res”:”Sorry, please enter the key”}

I don’t understand why this error message is showing as I have passed the key!