Yii2 - How to write OAuth Client - oauth-2.0

Question

I am trying to add FitBit OAuth to my Yii2 app. I have successfully managed to setup other APIs, such as Google and Facebook - but these are using yii\authclient\clients. Here I am trying to write my own using yii\authclient\OAuth2. Here is the code I have:
<?php
namespace app\auth\clients;
class Fitbit extends \yii\authclient\OAuth2{
public $authUrl = 'https://www.fitbit.com/oauth2/authorize';
public $tokenUrl = 'https://api.fitbit.com/oauth2/token';
public $apiBaseUrl = 'https://api.fitbit.com';
public $scope = 'profile';
protected function initUserAttributes(){
return $this->api('/1/user', 'GET');
}
protected function defaultName(){
return 'fitbit';
}
protected function defaultTitle(){
return 'Fitbit';
}
}
Here is the configuration I have:
'authClientCollection' => [
'class' => 'yii\authclient\Collection',
'clients' => [
'google' => [
'class' => 'yii\authclient\clients\Google',
'clientId' => 'ID',
'clientSecret' => 'SECRET',
],
'fitbit' => [
'class' => 'app\auth\clients\Fitbit',
'clientId' => 'ID',
'clientSecret' => 'SECRET',
],
But I am stumped, as this is the error I get:
{"errors":[{
"errorType":"invalid_request",
"message":"Authorization header required. Visit https://dev.fitbit.com/docs/oauth2 for more information on the Fitbit Web API authorization process."
}],
"success":false}
Any help would be really appreciated. Thank you.
EDIT My current thinking is something along the lines of:
return $this->api('/1/user', 'GET', [], [
'WWW-Authenticate' => 'Authorization: Basic ' . base64_encode($this->client_id . ':' . $this->client_secret),
]);
But this still gives me the same error.