Tesla do not offer a towing option for the Model S, or offer a tow hook or tow hitch. Tesla did not even submit the car for "type approved" for towing and therefore it is generally considered against law to do so. There are however some after market products that can be fitted that either allow equipment such as bike racks that require a tow hook/tow hitch to be fitted, or offer a full blown tow hook/tow hitch that they claim to have had approved by the local authorities.
Some of the after market tow hooks/tow hitches come without the electrics and some do, the price varying accordingly.
Carrying a bike rack using a tow hook is an increasingly common activity and the tow hook is no more than a suitable mounting point for the rack in much the same way as roof boxes can be fitted to cars for extra storage.
One important consideration with respect to the tow hook is the need for an electrical connector which enables external lighting to be controlled like the rear lights of the car. This is obviously important as any bike rack and bikes can obscure the cars lighting which would be dangerous if not illegal. Some of the cheaper options do not offer electrics and any driver using them needs to satisfy themselves of the requirements regarding the rear lighting.
As towing was not type approved by Tesla for the Model S, in general towing is not regarded as an option. Some regions take a more liberal view and people tow with their model S, and some owners ignore the legalities and take the risk. We do not support illegal use of Teslas and owners should satisfy themselves on the local requirements.
To address the issue, some European companies have created and tested a towing option which they claim has been tested to EC Council Directive 94/20 / EC, the required legislation, and offer an individual certificate from TÜV Süd as part of the purchase. An example is MisterDotCom in Germany. This option includes the required electrical connections and they have taken the trouble to submit their product to compliance testing and may appear more expensive than others. While we have no reason to doubt the veracity of their claims, it is still not clear if this meets all the required testing requirements to be legal, but it would give the owner of a modified Model S some confidence that the solution is fir for purpose. We strongly recommend talking to them and asking for assurances from them for your country.
Because Tesla do not approve any towing option they have not published any specification on towing limits or towing capacity. It is worth noting that the Model X and Model 3 specification and limits for towing are considerably different and in the case of the Model 3 are quite limited. Range is also significantly decreased when towing and owners of Model X who have towed a caravan have reported range has been reduced to as low as 40% of the normal range. Supercharging with a trailer or even a bike rack can also be a challenge.
The general principles or warranty worldwide is that a manufacturer can only avoid a warranty claim if any unofficial change has a direct impact on the part being covered by warranty. A simple example would be if an owner decides to replace the brakes on their car, a subsequent issue with any braking component could be linked to the change and therefore not warranted.
A tow hook or tow hitch could be attributed to a range of potential issues including rear suspension, air suspension, bodywork, subframe, increased motor loading, etc etc. Tesla attitude to warranty is changing with an increasing willingness to avoid warranty claims if they suspect the owner has caused the problem in some way. We can not offer any definitive answer on the impact of warranty but owners should be aware of the potential issues that present themselves in the future.