Not all of our UPS' have service tags. Only UPS' manufactured in the last couple of years have service tags. If your UPS has a service tag then it is usually on a pull out tab.
The warranty period on a Dell UPS is 3 years from date of purchase or if it is purchased on the same order as a rack it will be warrantied for the same period as the rack, whichever is greater. The purchase date is verified by the order number of the purchase. Rather than providing support with a service tag you will provide them with the order number.
Dell EMC, Enterprise Engineer