$tocken = 'ключ';
$url = 'https://api.ok.ru/graph/me/subscribe?access_token='.$tocken;
$result = file_get_contents($url, false, stream_context_create(array(
'http' => array(
'method' => 'POST',
'header' => 'Content-type: application/json',
'content' => '{"url": "https://твой сайт/webhook.php"}'
)
)));
print_r($result);
{"success":true}