Question: How to use Client Credentials and Refresh Token. Possibly missing Signer?

[ryan-k-t]

I'm trying to get an API client to authenticate following your instructions but keep running into issues. I think it may have something to do with a present RefreshToken which I know you've mentioned that this isn't how the RFC defines the appropriate process here but I'm wondering if my issue is different.

I'm trying to consume an API with what I assume is ClientCredentials using a RefreshToken. I can get a proper Access Token using cURL like so:

curl -X POST https://domain.com/API/Login/oauth2 
     -H "Content-Type: application/x-www-form-urlencoded" 
     -H 'Authorization: Bearer XXXRefreshTokenXXX' 
     -d "grant_type=client_credentials&client_id=ClientTest&client_secret=ClientSecret&redirect_uri=https://domain.com/Auth/OAuth2"

Now when I try to recreate that using the Guzzle OAuth2 Subscriber library I can't get it to authenticate. I keep getting a 500 error. The code I'm using is this:

$reauth_config = [
    "client_id" => "ClientTest",
    "client_secret" => "ClientSecret",
    "refresh_token" => "XXXRefreshTokenXXX",
   'redirect_uri' => "https://domain.com/Auth/OAuth2",
];
$reauth_client = new GuzzleHttp\Client([
    // URL for access_token request
    'base_uri' => "https://domain.com/API/Login/oauth2",
]);

$grant_type = new ClientCredentials($reauth_client, $reauth_config);
$refresh_grant_type = new RefreshToken($reauth_client, $reauth_config);
$oauth = new OAuth2Middleware($grant_type, $refresh_grant_type);
$oauth->setTokenPersistence($_token_storage);
$stack = HandlerStack::create();
$stack->push($oauth);

$client = new GuzzleHttp\Client([
    'handler' => $stack,
    'auth'    => 'oauth',
]);

$response = $client->get("https://domain.com/API/Member?MemberType=I&DisplayStart=0&DisplayLength=100");

From what I can tell it's not passing the Authorization Bearer along with the request. I've looked into the Signer portion of the library but I honestly can't suss out where to put that code. Is this just a matter of my noobness or something else?