Basically, if you have a 240v source with enough amps and the correct adapter, you are good to go. You can not charge the KTM off of 120v. In US, you'll typically use a 240v Dryer plug and us the adapter (makes more sense to wire up an outlet type that is one of the US standard 240v outlets than source a receptacle that will work directly with the KTM plug). If you are using a generator, same thing applies. Generator must have a 240v plug and you must ensure the generator can provide adequate amps. The charger label should list max amp draw which would account for start up surge, if any. If generator is adequately sized, then use the appropriate adapter for your generators 240v plug and plug in the KTM charger.
For others to reference in the future:
https://chargethebike.com/how-to-ch.../how-to-charge-ktm-electric-dirt-bike-in-usa/It's a pain, but understandable that KTM is not making any special chargers for the US market since they are only selling these at a handful of KTM dealerships in very limited numbers ( around 100 of them in 2020). For the price, they could provide the adapter for a typical 240v plug that would be found here, but then, there are multiple types of 240v receptacles in the US (3 wire 240v, 4 wire 240v) so I get why they don't bother providing and adapter and leave that up the buyer. Long term, if they start shipping them regularly and to all dealerships, then we'll likely see a charger specific to the US (most likely the 4 wire 240v). And when they do that, ideally they would make it compatible with not only 240v but with 120v as well (again, would require an buyer-sourced adapter). Understood that the charge cycle will take much longer on 120v, but for those with easier access to 120v, it would make ownership and charging options much easier on the customer. KTM could perhaps do as Zero does and make the 120v the standard charger and offer the 240v as a optional "fast charger".