How to send description when registering a new lead

Hello,

I want to register a new Lead with first name, last name and description. First name and last name works fine using the /Leads/register endpoint but it seems to ignore the description data. What am I doing wrong here?

Here is my code:

$json = array('first_name' => $firstname, 'last_name' => $lastname, "description"=>$description);
$request = Requests::post($rooturl . "/Leads/register", $headers, json_encode($json));