AddJsonFormatters() missing after migration from .NET Core 2.2 to 3.0

You can use Microsoft.AspNetCore.Mvc.NewtonsoftJson NuGet package, and configure it in the Startup.cs:

services.AddMvcCore()    
    .AddNewtonsoftJson(o =>
    {
        o.SerializerSettings.Converters.Add(new StringEnumConverter());
        o.SerializerSettings.ContractResolver = new CamelCasePropertyNamesContractResolver();
    })

I just found out that IdentityServer4 are slowly updating their samples for .NET Core 3.0. Attached is the link to their newer version of the code for the part you are asking about, hope it helps. https://github.com/IdentityServer/IdentityServer4/blob/master/samples/Quickstarts/1_ClientCredentials/src/Api/Startup.cs