Open the appsettings.json file, select the ConnectionStrings JSON key (refer to the following) and copy it to the clipboard:

[...]

"ConnectionStrings": {
  "DefaultConnection": "Data Source=(localdb)\MSSQLLocalDB;Initial Catalog=TestMakerFree;Integrated Security=True; MultipleActiveResultSets=True"
},

[...]

Right after that, open the appsettings.Development.json file and paste it right before the Auth key, as follows:

{
  "ConnectionStrings": {
    "DefaultConnection": "Data Source=(localdb)\MSSQLLocalDB;Initial Catalog=TestMakerFree;Integrated Security=True; MultipleActiveResultSets=True"
  },
  "Auth": {

[...]

Once done, go back to the appsettings.json file and replace the now copied ConnectionStrings > DefaultConnection value with the following:

"Server=localhost\SQLEXPRESS;Database=TestMakerFree;User Id=TestMakerFree;Password=SamplePassword123;Integrated Security=False;MultipleActiveResultSets=True"

Needless to say, replace SamplePassword123 with the password you've set for the TestMakerFree login a short while ago.