From Solution Explorer, open the appsettings.json file and add the following (new lines highlighted):

{
  "ConnectionStrings": {
    "DefaultConnection": "Data Source=(localdb)\MSSQLLocalDB;Initial 
     Catalog=TestMakerFree;Integrated Security=True; 
     MultipleActiveResultSets=True"
  },
  "Logging": {
    "IncludeScopes": false,
    "Debug": {
      "LogLevel": {
        "Default": "Warning"
      }
    },

[...]

This is the connection string we’ll be referencing in our project's Startup.cs file later on.