I see the trial ad every time I visit youtube for the first time of the day. After that, it will not show up again for the rest of the day. That's why I thought uBlock worked last time, b/c I was not seeing them until the following day.
I do not keep my account logged in. This might be another factor. But I don't know.
Thanks for the recommendation. I've been using Tampermonkey for a while. It's an incredibly useful extension and I use it everyday. Since the youtube tv ad pops up only for the first time of the day, I was thinking maybe Keyboard Maestro is a better choice (as I need to execute the javascript only once a day? I don't need it to execute every time I visit youtube).