As of the time of this post, it doesn't appear that the scaffold-dbcontext command supports a appsettings.json connection string lookup. In other words, you must explicitly type the full connection string using the scaffold-dbcontext cli syntax:
Scaffold-DbContext -Connection "Server=(localdb)\ProjectsV13;Database=MyDbName;Trusted_Connection=True;" -Provider Microsoft.EntityFrameworkCore.SqlServer -OutputDir Model -Context "MyDbContextName" -DataAnnotations -Force -Project MyEntitiesProject -StartupProject MyEntitiesProject
Not directly related to the OP, but just as a word to the wise... using the StartupProject option is helpful so you don't have to switch the Startup Project in Visual Studio to your entities project (e.g., MyEntitiesProject) in order to run the scaffold-dbcontext command.
There's a good link detailing the scaffold-dbcontext command options here.
Also be sure to have the following packages installed in your project in order to use the scaffold-dbcontext command in the nuget package manager console:
Install-Package Microsoft.EntityFrameworkCore.SqlServer
Install-Package Microsoft.EntityFrameworkCore.Tools
Install-Package Microsoft.EntityFrameworkCore.Design