Ps, extension leads will differ in diameter so don't slam it **** but if it's just a DIY extension then it should go gently, opening the boot you will have to do by the lock in the trim above the number plate light, it can be stiff but it should unlock the boot without power
PPS sorry didn't read the kitchen window bit, but some windows have the slightly ajar latch that still keeps the window closed but with an air gap and probably enough to leave the cable coming through.