Add the connection string to a modern .NET Core application at the appsettings.json file.

"ASP.NET Core 2 and Angular 5" by Valerio De Sanctis suggests leading everything else like so:

{
   "ConnectionStrings": {
      "DefaultConnection": "server=.\\JAESCHKE;database=Packt;Integrated
            Security=true;"
   },
   "Logging": {