Generally this information is left in the truck by the U-Haul company.
If you are unable to locate it, just give the U-Haul folks a call and they will be able to access that information through their records.
However, key replacement for rental trucks is actually fairly common so the key may have already been replaced by a former lessor.
If you are unable to get a new key, the truck will need to be towed to the nearest U-Haul location for them to make a replacement key.
This will significantly increase the cost of your move since you will need to pay for the tow as well as whatever U-Haul decides to charge for the replacement key.