First, use the steps you found and get your original remote working with the code.
Second, learn the code in Roomie as its own device with just that one code:
roomieremote.com/faq/#learnir
Third, add the newly learned device as an infrared device to your Room.
Finally, open your Comcast Activity, tap the Design Name at the top to edit the Remote Design, choose a button you'd like to send that command, and add/change the command for that button to send the command you learned to the new device.
For this device specifically, you may also want to attach your RoomieCodes.plist file here after you learn the code for future users that come along and read this thread so that they can skip steps 1 and 2.
Thank you.