Custom JSON serializer not working

public class ExternalId
{
    public string Id { get; set; }
}

{
    "id1": "a",
    "id2": "b",
    "id3": "3"
}
// to e.g. this class
public class MyDTO {
    public ExternalId Id1 {get; set;}
    public ExternalId Id2 {get; set;}
    public ExternalId Id3 {get; set;}
}

public class ExternalIdSerializer : JsonConverter<ExternalId>
{
    public override ExternalId? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        return reader.GetString() is null
            ? null
            : new ExternalId(reader.GetString()!);
    }

    public override void Write(Utf8JsonWriter writer, ExternalId externalId, JsonSerializerOptions options)
    {
        writer.WriteStringValue(externalId.Id);
    }
}

services
    .AddControllers()
    .AddJsonOptions(options =>
    {
        options.JsonSerializerOptions.PropertyNameCaseInsensitive = true;
        
        var converters = options.JsonSerializerOptions.Converters;
        converters.Add(new JsonStringEnumConverter());
        converters.Add(new ExternalIdSerializer());
    });