PHP - POST array data with Guzzle or oauth2-client

Question

I need some help connecting to deviantArt's API for whois.

I am building a Laravel app that needs to connect to deviantArt's API.

I am able to successfully connect to the API using Socialite, The League's OAuth2-Client, Guzzle and cUrl, but only for a whoami request.
GET /user/whoami

I can make a successful whois connect, but only using cUrl.
POST /user/whois

curl https://www.deviantart.com/api/v1/oauth2/user/whois \
-d "usernames[0]=justgalym" \
-d access_token=Alph4num3r1ct0k3nv4lu3

I'd like to stay using as much of the OAuth2-Client plugin as possible, but at this point, I am just looking for something that works.

My best guess is that I am not sending over the POST variables correctly.

Attempt 1 - Using Guzzle Request

$options = [
  'form_params' => [
    'usernames[0]' => 'justgalym',
    'expand' => 'user.profile',
  ],
  "Authorization" => "Bearer ".$this->token->getToken()
];

$request = new Request('POST', $url, $options);

Attempt 2 - Using Guzzle Request

$options = [
  'form_params' => [
    'usernames[]' => 'justgalym',
    'expand' => 'user.profile',
  ],
  "Authorization" => "Bearer ".$this->token->getToken()
];

$request = new Request('POST', $url, $options);

Attempt 3 - Using Guzzle Request

$options = [
  'form_params' => [
    'usernames' => 'justgalym',
    'expand' => 'user.profile',
  ],
  "Authorization" => "Bearer ".$this->token->getToken()
];

$request = new Request('POST', $url, $options);

Attempt 4 - Using Guzzle Request

$options = [
  'form_params' => [
    'usernames' => 
       [0 => 'justgalym'],
    'expand' => 'user.profile',
  ],
  "Authorization" => "Bearer ".$this->token->getToken()
];

$request = new Request('POST', $url, $options);

Attempt 5 - Using Oauth-client getAuthenticatedRequest

$request = $this->provider->
           getAuthenticatedRequest(
               AbstractProvider::METHOD_POST, 
               $url, $this->token,  ['body' => 'usernames[0]=justgalym'
]);

Attempt ...

and many others

Result for all attempts give me the same error.

array(
  'error' => 'invalid_request', 
  'error_description' => 'Request field validation failed.', 
  'error_details' => 
    array(
      'usernames' => 'usernames is required'
    ), 
  'status' => 'error'
)