Adding UWP plugins to Unity3D

I wrote all my custom Unity3D components as plugins with the idea that first I would deploy the plugins into Unity3D, then be able to attach components to GameObjects based on the types within my plugins.

I wrote a plugin that has a MonoBehavior based component. I built the class library as a UWP library but Unity3D won't process it. I dropped that plugin assembly into my Unity3D project folder. However, when I use the editor to try to attach the component in the plugin assembly onto the GameObject, Unity3D just shows other scripts (not the ones compiled into the plugin).

How can I get the components in my plugin to show as possible new scripts in the add component wizard?

 

I followed these instructions:
https://docs.unity3d.com/Manual/windowsstore-plugins.html