The leasing company’s standard answer will be that the car needs to be returned in the same condition it was received, which means if you fit a towbar (or anything else for that matter) you should remove it before handing the car back, and return the car to its original condition.
I fitted a towbar to a company car, and paid for everything. When it came time to give it back I asked if they wanted me to remove it like they said, but was told not to bother as it would leave unsightly holes, exposed wires, etc.
So, as with many things in life, there’s a whole grey area between ‘the policy’ and common sense, and where you fall between the two will depend on who you talk to on the day.