When an app is installed for the first time on a device, a file named Installation.Token will be generated automatically in the resources folder. Its content will be a generated Guid.
To access the token at runtime, you can call UIRuntime.GetInstallationToken().
It is a good idea to send this token, along with the Device.Platform value to the server when logging in or registering a user.
As each user can potentially access your platorm from multiple devices, on the server app create a new entity type named UserDevice with the properties of: