您可以编写支持编写字典键的自定义转换器：

class JsonEnumNumberConverter : JsonConverter<MyEnum>
{
    public override MyEnum Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) => throw new NotImplementedException();

    public override void Write(Utf8JsonWriter writer, MyEnum value, JsonSerializerOptions options) => throw new NotImplementedException();

    public override void WriteAsPropertyName(Utf8JsonWriter writer, MyEnum value, JsonSerializerOptions options) =>
         writer.WritePropertyName(value.ToString("D"));
}

示例用法（或传入Serialize方法的设置）：

[JsonConverter(typeof(JsonEnumNumberConverter))]
public enum MyEnum
{
    One = 1,
    Two = 2
}

或者使用Macross.Json.Extensions中的JsonStringEnumMemberConverter，它扫描EnumMemberAttribute：

[JsonConverter(typeof(System.Text.Json.Serialization.JsonStringEnumMemberConverter))]  
public enum MyEnum
{
    [EnumMember(Value = "1")]
    One,
    [EnumMember(Value = "2")]
    Two
}