Authentication Problem - Error 405 Method Not Found

I am trying to get my authentication set up in Rose Rocket and I am following this documentation: https://roserocket.readme.io/docs/getting-started
I was able to download the Rose Rocket API collection and environment into postman, and I filled in the Body of the request with the correct information (username, password, client_id, and client_secret); however, I am getting a "405 Method Not Allowed Error" when sending. Any idea of the problem here?