Ads before and inside a video arrive through requests AdSilence knows. They are in the lists that are switched on out of the box.
You do not have to add a list or write a rule. What AdSilence brings for YouTube is active right after installing.
Besides the ads, a video page runs measurement points. AdSilence catches them from the same rule set — it is not only about what you see, but about what leaves you.
YouTube reworks things regularly. The lists follow, and if a page ever breaks, switch AdSilence off for that one page — the exception applies there only.