Okay. So, I wasn’t exactly sure about the required params either, so I started digging into the spark-cli source code to figure it out. I found this:
https://github.com/spark/spark-cli/blob/master/js/lib/ApiClient.js#L133
Which told me all I needed to construct a curl request to illustrate the endpoint in action:
curl -X POST https://api.spark.io/oauth/token \
-d 'username=<YOUR_PASSWORD> \
-d 'password=<YOUR_PASSWORD>' \
-d 'grant_type=password' \
-d 'client_id=<WHATEVER>' \
-d 'client_secret=<WHATEVER>'
the client_id and client_secret can be whatever you want.
Hope that helps. Happy Hacking.
-joe