In the Quick start guide

NOTE: The HomeKit code is unique to each Wemo WSC010 unit. If you purchase multiple Wemo WSC010 units and use the code in the Quick start guide, make sure it matches the unit you are setting up.
On the back of the Wemo WSC010

NOTE: You will first need to remove the battery cover to view the code on the back of the Wemo WSC010.
Related Articles:
What is needed to set up the Wemo WSC010
Installing and replacing the battery in the Wemo WSC010