Normally the client_id and client_secret are passed in the Authorization header, and the grant_type is passed as a body param. So the curl command would look like this:
curl -X POST -u client_id:client_secret -d 'grant_type=client_credentials' https://org-env.apigee.net/oauth/client_credentials/token
That's the normal case. Try it. If it does not work, then your API Proxy may be expecting something different. We cannot tell unless you show us the proxy flow and the GenerateAccessToken policy.
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