I would go with a block heater. I would think your local dealer can help you with that.
Looking at the picture you supplied on the switch you need to push the key in for the pre-heater or glow plugs to come on. On my tractor you push the key in when it is in the run position. I do a 10 count let the key out and then start it. I can tell it is working because the lights on the dash dim. The manual also says if after starting if the engine is running rough to push the key in again. Give this a try and see, if it works for you.
It appears from viewing JD parts a glow plug can be inserted into air intake pipe. Parts key 10 DZ102076 Coolant Heater - COOLANT HEATER, WITH CORD KIT View/Update 108.37 USD
I just installed a block heater in my JD4300 that I purchased right from the JD dealer. It looks like a frost plug heater for a car or truck except it's threaded on the outside. To install it, (mine has a Yanmar 3 cyl diesel engine) you unscrew a large pipe plug on the left side of the engine, just above the hydraulic pump, and then screw the heater in where that pipe plug was. It works great, she starts like it's summer. I also use AMSOIL synthetic oil in the engine. I live in northern Mn. It's been below zero here lately, and the tractor starts after being plugged in for an hour or so.