Setting up authentication token retrieval in Postman API

Has anyone had luck setting up token retrieval in Postman API?

I can get the token when going through the folder authorization steps, but not when trying to create POST/GET requests. Using the "Get New Access Token" method in the folder authorization, I am required to login every time. This is not sustainable.

For anyone with experience with this, I am attempting to follow the steps here.

1. In Postman, I create a GET request. GET https://api.cc.email/v3/idfed?

Params:

client_id: {{clientid}} (same as in the Get New Access Token process)

redirect_uri: https://localhost (same is in the Get New Access Token process)

response_type: code

scope: contact_data+campaign_data

2. This returns an HTML form that does not seem to work in Postman.

3. I am expecting to get an authorization code, but I do not see where this comes from. 

4. Attempting to send a POST request to the https://idfed.constantcontact.com/as/token.oath2?code=????&redirect_uri=https%3A%2F%2flocalhost&grant_type=authorization_code

Headers: 

Authorization: Basic (base64 {{clientid}}:{{API Secret}})

Content-Type: application/x-www-form-urlencoded

Response is:

{
    "error_description": "unable to parse Authorization header javax.security.auth.login.LoginException: No credentials provided with BasicAuthorization",
    "error": "invalid_client"
}