OkHttp POST sending invalid format to Pocketbase

I'm trying to send a POST request via OkHttp to my local Pocketbase server, but it's not working.

In Pocketbase, I have a collection called servers. I have a field called guild_id which is set to the Number type. For now, the access is admin-only. I'm using OkHttp with Java to try to add a new record. Here is the code:

OkHttpClient client = new OkHttpClient();
final String url = "http://localhost:8090/api/collections/servers/records";
final long serverId = 123L;
final String token = System.getenv("token");

RequestBody body = new FormBody.Builder()
        .add("guild_id", String.valueOf(serverId))
        .build();

Request r = new Request.Builder()
        .url(url)
        .addHeader("Authorization", "Bearer " + token)
        .addHeader("Content-Type", "multipart/form-data")
        .post(body)
        .build();

try (Response resp = client.newCall(r).execute()) {
    if (!resp.isSuccessful()) {
        System.err.println("Error posting to pocketbase: " + resp.body().string());
    } else {
        System.out.println("Request successful. Response: " + resp.body().string());
    }
} catch (Exception e) {
    e.printStackTrace();
}

When I run this code, it always gives the same error:

Error posting to pocketbase: {"code":400,"message":"Failed to load the submitted data due to invalid formatting.","data":{}}

I tried to send the same request via Postman, and it worked. I'm really not sure what's up, I suspect it might be the way Java and/or OkHttp handles requests? Anyway, does anyone know how I might fix this?

Okay, I figured it out (thanks to Thomas W's comment). Courtesy of Wireshark, I found out that OkHttp was sending an application/json content type instead of multipart/form-data.

To fix this, I used MultipartBody.Builder instead of FormBody.Builder and specified the type like this:

RequestBody body = new MultipartBody.Builder()
        .setType(MultipartBody.FORM)
        .addFormDataPart("guild_id", String.valueOf(serverId))
        .build();

If you're curious...

Here is OkHttp's old request:

vs Postman's: