How to correctly reference and include a C++ DLL in a C# project's Nuget package?

Question

I have a Visual Studio solution which contains a C++ project, which I need to reference in another C# .NET Core project contained within the same solution. The C# project here is supposed to output a Nuget package which needs to contain the DLL from the reference C++ project, and I can't figure out how to do this :/

In the C# project's CSPROJ file I have a reference to C++ project like this:

  <ItemGroup>
    <ProjectReference Include="..\Core\Com.BaseName.Core.csproj" PrivateAssets="All" />
    <ProjectReference Include="..\Hashing\Com.BaseName.Hashing.vcxproj">
      <Project>{028E03CD-5DAE-40FF-AAEF-4DF471D0076F}</Project>
      <Name>Com.BaseName.Hashing</Name>
      <ReferenceOutputAssembly>true</ReferenceOutputAssembly>
      <OutputItemType>Content</OutputItemType>
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
      <Private>true</Private>
      <CopyLocalSatelliteAssemblies>true</CopyLocalSatelliteAssemblies>
    </ProjectReference>
  </ItemGroup>

(Sidenote: the first project reference above is another C# project within the same solution)

But when I generate the Nuget package it doesn't contain C++ project's DLL.

So then I tried adding the line marked with *** below in the the C# project's NUSPEC file:

<files>
  .........
  ***<file src="Com.BaseName.Hashing.dll" target="lib\native" />
</files>

But then I get this error when trying to generate the nuget package:

Some target frameworks declared in the dependencies group of the nuspec and the lib/ref folder do not have exact matches in the other location. Consult the list of actions below:
- Add a dependency group for native0.0 to the nuspec

Currently only the netcoreapp3.1 framework is mentioned in my NUSPEC file, as shown below:

<package>
  <metadata>
    ......other metadata here.....
    <dependencies>
      <group targetFramework="netcoreapp3.1">
      </group>
    </dependencies>
  </metadata>
  <files>
    <file src="Com.BaseName.Sdk.dll" target="lib\netcoreapp3.1" />
    <file src="Com.BaseName.Core.dll" target="lib\netcoreapp3.1" />
    <file src="Com.BaseName.Hashing.dll" target="lib\native" />
  </files>
</package>

(side note: the first DLL mentioned under the 'files' property above is of the C# project itself, and the second DLL is of another C# project that's referenced)

Can someone please tell me how to correct specify that the unmanaged DLL of the referenced C++ project needs to be present in the Nuget package too ?

(Also FYI, I've looked at a bunch of other related stackoverflow posts but none seem to address my question specifically.)