I couldn't find a way to set Youtube as the default app on startups (And I don't think there's an option to do so), so I ended up just try tuning the live TV and because there is no cable available, now it shows a black screen with "No signal" text on it. This solved my problem, so I don't see the need to set Youtube as the default app. If I "cast" videos from the Youtube app on my phone, the TV will automatically switch to the Youtube app.
WebOS is just a useless pos that shouldn't have been made default to a premium product like this. (The C9) .
Many people use either apple TV or G-cast or Amazon sticks as default input on startup. These are far more superior than webOS.