When I try to add a missing assembly to my project (for unit testing specifically) I noticed that the Assemblies tab is missing in the \"Add Reference\" dialog , see:
It is correct behaviour. The Assemblies tab is not available for Net Core projects. You wont be able to add assemblies from your system, because Net Core projects work with dependencies from Nuget. So, to add your references, you need to use the Nuget Package Manager instead.