If it was bought with a computer in the same order, you will need to provide the computer's Service Tag. If it was bought separately, you will need to provide the order number and a copy of the invoice. Details here: How to get support for your Dell Keyboard and Mouse that is still under warranty. 
How can I claim Dell mouse warranty. When i click help button it's  asking for service, express tag where Dell mouse doesn't have that.
Dell mouse warranty claim