2 choices
via a timer on the eManager
via a manual zap of the button on the CarNet website/app
I use a granny for my GTE and have successfully preheated for say 7am, when the min charge level was set to 80% to have the battery finish charging at 7ish too.
Both use cases, 1) a timer with heat ticked, or 2) having forgot to set a timer, plugged the car in at 6am and then zapped it from the CarNet app manually to heat, about 20mins before I want to leave.
Some people have reported poor preheating on a granny whilst still charging but I've not noticed it.
A car with defrosted windows, and immed drive-off-ability is a luxury during winter morning commutes.