Authorization code is malformed.

So I have a API Key and Secret with I ran in the terminal 

echo -n APPIKEY:ssecret | base64....I then put that into the Authorization header to the endpoint 

https://idfed.constantcontact.com/as/token.oauth2?code=code&redirect_uri=https://localhost&grant_type=authorization_code

in postman I set up a post to the above address with the base64 encoded API Key and secret to get the authorization key......what I get back in postman is