Примерно так:
require($_SERVER["DOCUMENT_ROOT"]."/bitrix/modules/main/include/prolog_before.php");
$user_id = 1;
$result = \Bitrix\Main\UserTable::getList(array(
'select' => array('EMAIL', 'LOGIN', 'NAME', 'LAST_NAME'),
'filter' => array('ID' => $user_id),
'limit' => 1
));
$arUser = $result->fetch();
$post_user = 'login='.$arUser["LOGIN"].'&name='.$arUser["NAME"].'&last_name='.$arUser["LAST_NAME"].'&email='.$arUser["EMAIL"];
$URL = 'https://domain.ru/handlers/file.php';
if( $curl = curl_init() ) {
curl_setopt($curl, CURLOPT_URL, $URL);
curl_setopt($curl, CURLOPT_RETURNTRANSFER,true);
curl_setopt($curl, CURLOPT_POST, true);
curl_setopt($curl, CURLOPT_POSTFIELDS, $post_user);
$out = curl_exec($curl);
curl_close($curl);
}