In case of .NET 5:

services.AddControllers()
    .AddJsonOptions(opts => opts.JsonSerializerOptions.Converters.Add(new JsonStringEnumConverter()));

In .net-core-5.0 and asp.net-core-5.0, Microsoft has added support for de/serializing enums via the JsonStringEnumConverter Class.

Decorate your enumeration's values like this:

using System.Runtime.Serialization;
public enum VipStatus
{
    [EnumMember(Value = @"IS_VIP")]
    VIP = 1,

    [EnumMember(Value = @"IS_NOT_VIP")]
    NonVIP = 2,
}

Given a class like this one:

class MyClass {
    public VipStatus MyVipStatus { get; set; }
}

You could use JsonStringEnumConverter inline to serialize an instance of the class like this:

using System.Text.Json;
using System.Text.Json.Serialization;
// ...

var myObjectWithEnums = new MyClass()
{
    MyVipStatus = VipStatus.NonVIP
};

var options = new JsonSerializerOptions();

// Configures serialization to allow strings to be accepted and auto-converted to enum values.
options.Converters.Add(new JsonStringEnumConverter());

var json = JsonSerializer.Serialize(myObjectWithEnums, options);
// serialized output is: { "myVipStatus": "IS_NOT_VIP"}

If you're using ASP.NET Core 5 then you could configure the app at startup to use JsonStringEnumConverter to serialize all incoming requests:

public async void ConfigureServices(IServiceCollection services) {
    // ...
    services
        .AddControllers()
        .AddJsonOptions(options => {
            // Configures serialization to allow strings to be accepted and auto-converted to enum values.
            options.JsonSerializerOptions.Converters.Add(new JsonStringEnumConverter());
        }
    // ...
});

More reading: How to serialize and deserialize (marshal and unmarshal) JSON in .NET Core. If you're working in ASP.NET then this is also of interest: Web defaults for JsonSerializerOptions.