How to send empty array param with Retrofit

test.java

// Create RequestBody
FormBody.Builder builder = new FormBody.Builder();
List<String> additionalFields = ...;
if (additionalFields.isEmpty()) {
    builder.add("additional_fields[]", "");
} else {
    for (String additionalField : additionalFields) {
        builder.add("additional_fields[]", additionalField);
    }
}

// Send
Api.editApi(builder.build()).enqueue(callback);

// API
interface Api {
    @PUT("put_api")
    Call<Void> editApi(@Body RequestBody body);
}