Those are set from environment variables, which can be set in a number of ways. The simplest of which is editing your bin/hubot and adding:
export HUBOT_DISCOURSE_USERNAME=myuser
export HUBOT_DISCOURSE_KEY=secret
export HUBOT_DISCOURSE_SERVER=https://discourse.example.com/
before the exec line.
Bear in mind that the discourse key is a password, and so this file (or any env file) should be treated as secret and kept safe.