The vehicle must be able to activate the EVSE when it wants, or battery preconditioning won't happen...IOW, it must not only be plugged in, whatever control you have on the EVSE must allow it to apply charging power to the car. That means, you would need to enable the EVSE to be on at least 3-hours prior to your departure time, and probably 3.5-hours would be better if it's cold out. The car won't need the full amount of time when the weather warms up.
I've seen mine pulling over 20A when it's cold out while also preconditioning the cabin. The battery heater is 1000W, or just over 4A of that, so over 16A peak to condition the cabin. That's probably an extreme example.
The car can turn on cabin conditioning regardless of the EVSE being attached, but not the battery conditioning.
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)