The manufacturer needs to supply it.
(Or, the device would need to give you a way to program it, but that’s just an extra burden for you. So, the manufacturer needs to give it to you, if it’s not already in some documentation. It should typically not be printed on a label on the device, as it’s a secret and people tend to forget to remove labels.)
Aside, maybe it’s the same for you too:
That’s very bad though: even the AppEUI should be unique for off-the-shelf devices.