sso question

Hello.

Regarding https://docs.whmcs.com/WHMCS_Single_Sign-On_Developer_Guide

1. The application requests an access token by sending its credentials to the OAuth Token API Endpoint

$post_data = Array(
'code' => $code,
'client_id' => $client_id,
'client_secret' => $client_secret,
'redirect_uri' => $redirect_uri,
'grant_type' => 'authorization_code',
//'grant_type' => 'single_sign_on',
);

$curl = curl_init($whmcs_token_url);
curl_setopt($curl, CURLOPT_HEADER, false);
curl_setopt($curl, CURLOPT_POST, true);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_POSTFIELDS, $post_data);
$result = json_decode(curl_exec($curl));
curl_close($curl);

var_dump($result); die();

Where to get $code?