How do I add a reference to a Shared Code project (.shproj) from another project

In TargetProject.csproj file add that string:

<Import Project="..\YourSharedProject\YourSharedProject.projitems" Label="Shared" Condition="Exists('..\YourSharedProject\YourSharedProject.projitems')" />

Visual Studio 2017:

Right-click the References or Dependencies item in the Solution Explorer and choose "Add Reference..."

The Reference Manager will open. Click "Shared Project" on the left side of the Reference Manager

Then select your project and click OK.

Adding the reference will require editing the project files where you want to add it. If it helps, you can peek at the project file where it is already referenced to see a working example.

Near the bottom of the project file (ex, a .csproj) there is likely already an <Import> element such as

<Project ...>
  [...]
  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
</Project>

You add the Shared project by adding another element like that for the Shared project. For example:

<Project ...>
  [...]
  <Import Project="..\Shared\Shared.projitems" Label="Shared" />
  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
</Project>

It is important for the Label attribute to be set to "Shared." If you set it to something else it will not be recognized as a Shared project by Visual Studio and will not appear under References. Project should be set to the path to the appropriate ".projitems" file.