How to use Power BI Rest API without GUI authentication (redirect uri)

Thank you skaratela for the most concise write-up yet on how to actually get access to the Power BI API. However I am still banging my head on the wall trying to simply get my groups listed in Postman. I have tried many iterations, including following your process exactly. In all cases I am getting a 403 Forbidden on API calls, even though I have can authenticate and get an access token.

I have tried a Web App in AAD using a client_secret and grant_type=client_credentials; I have tried a Native App using grant_type=password (because AAD Native apps no longer give you a client_secret to work with). I have my master account as the owner of both AAD apps, and full Power BI API permissions granted to them.

My most recent authorization call/response looks like this, using an AAD Native App owned by my master account as the Client:

POST to https://login.windows.net/{{tenantId}}/oauth2/token

{
"grant_type":"password"
"client_id":"xxxxxxxxxx"
"resource":"https://analysis.windows.net/powerbi/api"
"username":"[email protected]"
"password":"xxxxxxxxxxx"
"scope":"openid"
}

RESPONSE:

{
    "token_type": "Bearer",
    "scope": "Capacity.Read.All Capacity.ReadWrite.All Content.Create Dashboard.Read.All Dashboard.ReadWrite.All Data.Alter_Any Dataset.Read.All Dataset.ReadWrite.All Group.Read Group.Read.All Metadata.View_Any Report.Read.All Report.ReadWrite.All Tenant.Read.All Workspace.Read.All Workspace.ReadWrite.All",
    "expires_in": "3599",
    "ext_expires_in": "0",
    "expires_on": "1520882770",
    "not_before": "1520878870",
    "resource": "https://analysis.windows.net/powerbi/api",
    "access_token": "...",
    "refresh_token": "...",
    "id_token": "..."
}

Then I take that access_token, and use it to try and get my groups:

GET https://api.powerbi.com/v1.0/myorg/groups
Authorization: Bearer {{access_token}}

And I get a 403 error. Same result when I make the initial token request using an AAD Web App and providing client_id and client_secret parameters. Again, both my AAD apps have full Power BI API permissions granted, and are owned by my master account.

I have spent many hours struggling with this. Any help would be greatly appreciated.