Flight plan authorization

Hello, I have a small doubt, in controlled airports, and in light aircraft type C172, C337, Beechcraft Bravo, Beechcraft mooney, etc.

At what time is authorization requested for a flight plan, whether VFR or IFR, traffic pattern, etc.

I see some information on YouTube that it is done after starting the engines, but some say that it is always done before starting the engines.