Startup.cs

// ASP.NET Core 2.2 and earlier
public void ConfigureServices(IServiceCollection services, IHostingEnvironment hostingEnvironment)
{
    services.AddMvc()
        .SetCompatibilityVersion(CompatibilityVersion.Version_2_2)
        .AddJsonOptions(options =>
        {
            options.SerializerSettings.Formatting = hostingEnvironment.IsDevelopment() ? Formatting.Indented : Formatting.None;
        });
}