Ideally drill the wall but if you have to drill the the window frame use a high speed steel drill bit as it drills through plastic like a hot knife through butter, you may also come across some steel reinforcements in the frame so it will be good for that too. Put a loop on the cable outside the window. It will help stop any moisture that runs down the cable coming in the hole you drill as it will gather at the bottom of the loop and fall to the ground before it goes through the hole. Make sure you mastic. I’d be tempted to just mastic inside because water always finds a way so you need to give it a path of least resistance to stop it gathering where you don’t want it too, if you only mastic the inside you’ll in essence be creating a run off to the outside. But I’m undecided if that’s the best idea in this case as the frame will be hollow at least in some part and the danger is the section below where you’ve drilled fills with water and might escape from the fixings that hold the window in place causing damp spots where you don’t want them.
) due to the built-in bedroom furniture. I hope the bring the cable into the void behind it.
