Can you share exactly how you did this? If you use Basic auth in wM IS, you’d pass the credentials within the auth document and only the grant_type field under args. There would not be anything named “client_authentication”.
What might help is if you review the Console, and perhaps “Show raw log”, within Postman to see what it does. Based upon your screen shot, it should look something like this:
As can be seen here, Basic auth is used in the Authorization header, and the x-www-form-urlencoded form has just one field. To do the same in wM IS:
data/args/grant_type = client_credentials
auth/type = Basic
auth/user = (map from client_id)
auth/pass = (map from client_secret)
wM IS automatically sets the Content-Type.