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: https://www.base64encode.org/)

In post I am sending a post request
to: https://api.sandbox.freeagent.com/v2/token_endpoint

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:
http://baddotrobot.com/blog/2012/08/12/oauth-and-http-part-ii/
and
https://dev.freeagent.com/docs/oauth

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,
Konrad