Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
Per the instructions from the link you gave us, "You can access live streams of Golf programming on Golf Channel and NBC by authenticating with a cable, satellite, telco or live streaming TV subscription that includes the Golf Channel. There is no additional charge for this service, simply login using the Username and Password you use on your providers website. If you do not have, or do not know your Login information, please contact your service provider." I have this but cannot find the golf channel app to log in - Are you saying I have to load NBC sports app and then find golf channel through there?
Another option For iPhone or other Apple devices you can install iOS golf channel app then authenticate with cable provider and use new Roku AirPlay to show on tv. Note! Do not use iPhone screen mirror, that was very choppy for me but switched to using the cast button built in the app and all jitters went away ;-)