If you refer to the documentation,
AppDomain.GetAssemblies Method (System)[
^], it reads, "Gets the assemblies that have been loaded into the execution context of this application domain."
Adding a reference does not actually load the assembly. You would have to be using it in code somewhere for it to get loaded.
If you added a reference in your project then don't use Reflection to load it, just create a new instance of the class and go from there.