Webhook post request using Spark and Paypal Webhook Simulator for a Discord Bot

Question

I am trying to automate a Task for my Discord Server. The Task is that if someone donates me via paypal, my Discord bot should send a message and add the donator role to the discord user.

Well, I am currently trying to that using Paypal Webhooks, a webserver and an endpoint (using Spark).

Paypal can successfully send the webhook to my web-server from zap-hosting.com. But code wise, everything except the post request works out,

This is the code (I censored all private data):

package org.example;

import net.dv8tion.jda.api.JDABuilder;
import net.dv8tion.jda.api.OnlineStatus;
import net.dv8tion.jda.api.entities.Activity;
import spark.Spark;
import spark.Spark.*;


public class Main{

    private static JDABuilder jda;

    public static void main(String[] args) {

        jda = JDABuilder.createDefault("[Discord Token]");

        jda.setStatus(OnlineStatus.ONLINE);
        jda.setActivity(Activity.watching(" your bank account."));

        jda.build();

        webHookGathering webthread = new webHookGathering();
        Thread thread = new Thread(webthread);
        thread.start();


        Spark.port(8443);
        Spark.init();
        Spark.awaitInitialization();

    }

    public static class webHookGathering extends Thread
    {
        @Override
        public void run() {
            while (true)
            {
                try {
                    System.out.println("Gathering Started");

                    Spark.post("https://[Adress].com", (request, response) -> {
                        String webhookData = request.body();


                        System.out.println(webhookData);
                        response.status(200);
                        return "Webhook erhalten!";


                    });
                }
                catch (Exception e)
                {
                    e.printStackTrace();
                }

                try{
                    Thread.sleep(1000);
                    System.out.println("next");
                }
                catch (InterruptedException e)
                {
                    e.printStackTrace();
                }
            }
        }
    }

}

I created comments using sysout to see where my code gets and where not. This resulted that I was able to see that all the outputs are working except for the one in the post request.