Free Agent OAuth 2.0: HTTP Basic: Access denied

Hi Guys,

I’m struggling to ingrate with your api.

I keep getting:
HTTP Basic: Access denied.

I’m testing this through postman at the moment and I set the Authorize
header to:
Authorization: Basic ((<client_id>:<client_secret>) - all this is base 64
encoded string. This I generated from:

In post I am sending a post request

Content-Type: application/x-www-form-urlencoded;charset=UTF-8

In the body I have.
code: which I receive from the Authorization request.
redirection_uri: This is the same as the one I send in the query params of
the Authorization request.
grant_type: authorization_code

Also tried following this:

Could anyone give me a postman request or if you can see what I am missing,
let me know what I am missing, been stuck on this for hours?

Kind Regards,