Adding a reference to a project not working

Screenshot by Lightshot - VStudio. In the right you can see the Annotations reference added, down you can see that it has succeeded the built. Also you can see that UsedImplicitly(comming from JetBrains.Annotations) is found.

But unity doesn’t think so: Screenshot by Lightshot - error.

This is something very simple, but obviously I am doing smth wrong. What should I change?

It was an easy fix, pretty easy. Still not that obvious. I needed to add the .dll file to the project in unity itself. Unless I did this unity coudn’t find the reference that I had added previously in VS.