CustomDeviceId.cs

using Beamable.Api.Auth;
using Beamable.Common;
using Beamable.Common.Dependencies;
using UnityEngine.Scripting;

[BeamContextSystem]
[Preserve]
public class Testing
{
	[RegisterBeamableDependencies]
	public void Register(IDependencyBuilder builder)
	{
		builder.RemoveIfExists<IDeviceIdResolver>();
		builder.AddSingleton<IDeviceIdResolver>(() => new CustomDeviceIdResolver());
	}

}

public class CustomDeviceIdResolver : IDeviceIdResolver
{
	public Promise<string> GetDeviceId()
	{
		return Promise<string>.Successful("FAKE_DEVICE_ID");
	}
}