Including Dependent Assembly in NuGet Package in VS2017

I have a solution with two projects (NET Core). The first project (library) contains the main assembly, the other project (library) contains the assembly with resources, interfaces, and other stuff. The main project refers to the second project.

I am using Visual Studio 2017. When I create a NuGet package from the main project, the companion assembly is not included in the lib package folder. How can this be achieved?

Note. I don't want to reference the companion assembly as a NuGet package, I want this assembly to be part of the main package.