The frame number is typically stamped into the head tube. In the rare event that it is not, there is also a sticker with a barcode on it. Beneath that barcode you will also find your frame number.
You can find a video here that demonstrates how to find it.
You can fill out your warranty registration here.
You will need to locate the following serial numbers : Frame, battery, and motor
You will also need the purchasing information: Date of purchase, where you purchased, and invoice number.
*The video on the warranty page will also show you how to find your serial numbers.
Magnum guarantees the frame, motor, and electrical components are free of manufacturer defect for 1 year from purchase date. Please be sure to register your warranty on our website. Commercial or business use will void a bike’s warranty. The other components on our bikes are subject to the different manufacturers’ warranties. For example: Shimano, Schwalbe, and Tektro are commonly seen components on our bikes and a Magnum dealer can submit a warranty request to them on your behalf.