Go to Flashy dashboard to create an app: Integrations -> Create Mobile Push Notification App
After the creation you will get your APP ID.
Is the ID the messaging service has for the device, on Firebase it's called Registration ID.
When you create new contact on Flashy through the API you can pass the device token with the APP ID and it will assign it on the fly, the request will look like this (it works the same when updating contact).
You can subscribe contact to a list with a device token with the URL below: