not able to connect the watch to iphone and need to find the serial number. There is no serial number on the case of iwatch

My Apple watch does not connect to the iPhone and I need to talk to the representative to do that I need to book the appointment on the website, the website is asking for a serial number and says that I can get it from the apple watch app on the iPhone. When the apple watch does not connect to the iPhone then how can we get the serial number? I checked all the casing of the apple watch, but there is no serial number. Please let me know how to find it.

Thank you,