FYI, my Mother has the basic Xfinity tv cable box and the YouTubeTV App is found under menu, then Apps. Then you just log-in with your new account credentials (I assume they give you login credentials with a trial account?). However, I believe the Xfinity cable boxes vary from one region to another, so your box may be different than her Xfinity cable box, and possibly may not offer the YouTubeTV App.
I only get internet access from Xfinity and do not have an Xfinity tv cable box, but they include a free Xfinity streaming box with the service, which can be connect to any smart or non-smart tv, which also has the YouTubeTV App on it. I use YouTubeTV on the streaming box to watch all the Kings games on CA Comcast Sports channel, and/or TNT, or ABC. I usually record the local broadcast and the National broadcast and watch both of them eventually. The YouTubeTV App DVR has performed flawlessly for me all season long.
Hope you are able to get access for tonight's game!