How do I obtain an Access Token using a refresh token with OAuth2?

[TWagner]

I am  trying to make middleware work with a 2-legged OAuth flow. The server we are connecting to has already authorized our app, and we've been issued a refresh token (which doesn't seem to expire) that we can use to get new access tokens.

From the Service Providers Docs:

Authentication Workflow

The authentication workflow comprises of two steps:

1. Use your refresh token to get a client-scoped access token
2. Use the client-scoped access token to access resources

For each step, the Authorization header must be present.

The CURL Request would look like this (and it works perfectly on Postman

curl -XPOST {base_url}/v3/partners/oauth2/token \
-H "Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..." \ -F
-F "grant_type=client_credentials" \ -F
-F "client_id=daxko_api_user" \ -F
-F "client_secret=d9a2652cf96d734661c10d5ff2f8061f" \ -F
-F "scope=client:9999" 

I've hit a block on Step 1.

As an aside: I've used Postman obtain the Access_token... manually inserted that into kamermans solution and am able to retrieve data.  

I've gotten the closest with Kamermans /Guzzle-oath2-subscriber. However, it does not contain the first part of the connection process (e.g., getting the access token using the refresh token- and that deficiency is documented  as an issue https://github.com/kamermans/guzzle-oauth2-subscriber



I've also looked league/oauth2 .  But there isn't a shred of documentation for grant_type = ClientCredentials

Any pointers would be deeply appreciated!