import io.appwrite.Client;
import io.appwrite.coroutines.CoroutineCallback;
import io.appwrite.services.Messaging;

Client client = new Client()
    .setEndpoint("https://cloud.appwrite.io/v1") // Your API Endpoint
    .setProject("5df5acd0d48c2") // Your project ID
    .setJWT("eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ..."); // Your secret JSON Web Token

Messaging messaging = new Messaging(client);

messaging.createSubscriber(
    "[TOPIC_ID]", // topicId
    "[SUBSCRIBER_ID]", // subscriberId
    "[TARGET_ID]", // targetId
    new CoroutineCallback<>((result, error) -> {
        if (error != null) {
            error.printStackTrace();
            return;
        }

        System.out.println(result);
    })
);