Demo for how to enter AppD serial key? For that I have two suggestions.
1. If your AppD is hosted on vCD, that means you must have a AppD vApp template in the vCD catalog. In this case, best option is to simply instantiate a new AppD vApp instance from the catalog to showcase the firstboot process.
2. If AppD is a VM deployed locally on your ESX/WS/Fusion, best option is to take a snapshot before you start the firstboot process of the AppD VM. You could revert to that snapshot for showcasing the firstboot process.