How to handle sketches in Composer-based projects [duplicate]

I need to sketch out some behavior from a dependency (GuzzleHttpClient) installed via composer. How do I finagle my include_path in a sketch (located under <project root>/sketches) such that I can use GuzzleHttpClient in my sketch?

Just so you can see what I’m doing, here’s my sketch in its entirety.

<?php

use GuzzleHttpClient;

$client_id = 'my_client_id';
$username = 'my_username';
$password = 'bad_password';
$client = new Client;

$client->post(
    "https://my.sso.site/protocol/openid-connect/token",
    [
        'client_id' => $client_id,
        'username' => $username,
        'password' => $password,
        'grant_type' => 'password'
    ]
);

I want to see what exception gets thrown