I included the Unity3D Mixer interactive plugin. I added the interactivity manager prefab to my scene (albeit not the first GameObject in my scene). I have a script that calls MixerInteractive.GoInteractive() on the Start() method of a MonoBehavior. I build an UWP edition of my game, making sure the Microsoft.Mixer.CSharp.dll for UWP builds for Windows x64 as well. I run the UWP edition in Visual Studio 2017 Enterprise (running in Administrator role).

The Unity3d splash screen shows, then I get to the Mixer Go Interactive blue screen.

I am asked to input the six digit code below, but there is NO code on the screen!

This happened to me, and it was because I had accidentally checked the “Use a Secret Key” box when creating my OAuth Client ID. Recreated it without a Secret Key and everything connected no problem.

This is what the UWP app shows after the Unity3d splash screen is done displaying:

In addition to the screen, I noticed this error in my Visual Studio debug console output:
Exception thrown: ‘System.UnauthorizedAccessException’ in Microsoft.Mixer.Uwp.CSharp.dll