How do I get an access token using a refresh token? - oauth-2.0

Question

First, if this question is badly formed, I apologize.
In simple terms, the API server documentation that I'm trying to connect to provides this guidance:
Authentication Workflow The authentication workflow comprises of two steps:
Use your refresh token to get a client-scoped access token
Use the client-scoped access token to access resources
I'm stuck at step one.
I am using Guzzle 6.x and trying to authenticate via OAuth2.
Using Postman, I able to obtain the access token easily. I'd like to do this in my code using Guzzle. (or whatever it takes).
Postman generated the following PHP HttpRequest snippet. Can I do that with Guzzle? (does the question even make sense?)
<?php
$request = new HttpRequest();
$request->setUrl('https://api-tra/v3/partners/oauth2/token');
$request->setMethod(HTTP_METH_POST);
$request->setHeaders(array(
'cache-control' => 'no-cache',
'Connection' => 'keep-alive',
'Content-Length' => '559',
'Accept-Encoding' => 'gzip, deflate',
'Content-Type' => 'multipart/form-data; boundary=--------------------------158986490184667130446156',
'Host' => 'api-train.daxko.com',
'Postman-Token' => '28b9d8ac-d0ae-4036-9a61-202e9b39b220,f89cb196-9ada-4c1a-92f7-afc322fec03b',
'Cache-Control' => 'no-cache',
'Accept' => '*/*',
'User-Agent' => 'PostmanRuntime/7.19.0',
'Authorization' => 'Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpc3yYWluLWF1dGgiLCJzdWIiOiJmbWN3ZWJzaXRlX3RyYWluIiwiaWF0IjoxNTcwNTQ4OTE3LCJleHAiOjE1NzA1NDg5MTcsImFwaSI6Im9wc19hcGlfdjJfYXV0aCJ9.EnbGN121Ei5hlkPFT6Iqyd1h9_WXDGKplu5RsCH5QZs',
'content-type' => 'multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW'
));
$request->setBody('------WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="grant_type"
client_credentials
------WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="client_id"
fmcwebsite_train
------WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="client_secret"
blC}g!!E#{+_k.VLH
------WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="scope"
client:2069
------WebKitFormBoundary7MA4YWxkTrZu0gW--');
try {
$response = $request->send();
echo $response->getBody();
} catch (HttpException $ex) {
echo $ex;
}

Answer 1

You are using client credentials and in that flow refresh tokens are not typically used. Instead just send the access token to the API. If you use the token for long you may also need to check for a 401 response status when you call the API. If so then re-authenticate and call the API again with a new access token.