I haven't done this myself, but at one time I was planning to fit a tow bar so did investigate what was required.
From what I could find, Citroen had not generously fitted a helpfully-positioned socket at the rear of the car for a future trailer - at least not in my 2011 model - so you would have to crimp in to the existing lighting harness at the back.
What would also then be required is a new power cable to be run from a spare fuse in the fuse box behind the glovebox, and this is brought down towards the rear to supply the required power to the trailer lights via a 'bypass' relay such as https://www.ebay.co....veDSHXyLnW4gTMA .
The idea - as I understand it - is that the relay simply takes a small switching signal from the lights being deployed - tail, indicator, brake, fog etc - and uses the power from your new supply to actually power the lamps on your trailer. This way there is no additional drain on the car's existing lighting system, and also no confused signal for the car to wonder why its lamps are suddenly drawing more current.
So, there should be no reprogramming required.
That is the info I have gleaned, but please don't take it as gospel.