I had the dealer install a tow hitch on my 2016 when I purchased it. It was eight hour labor to install, dealer said it was labor intensive. I did not get the software that goes with the factory installed system.
I don't know any other than the official option, expensive though it is. However it might be worth trying a local welding shop as it is not rocket science.
For those who have not already purchased their Mk IV RR, on the Supercharged the factory tow package option is a real bargain as you get not only the Class III trailer hitch etc but the electronic locking rear diff. AL for about $1500 which I gather is not far off the price of having the dealer install a simple trailer hitch,