Trying to send a JSON-formatted HTTPS POST request from ESP32 results in Code 400

Question

Helping a friend out with an IoT project and I wanted to try having an ESP32-S3 send a JSON-formatted POST request to pushme.win, which is a super-simple Webhook push notification service. (IFTTT wanted to charge me for it). This would be triggered whenever a connected PIR motion sensor detects movement.

Assume that my unique Trigger ID is 101deadbeefcafe133769420. Using the command

curl -X POST https://pushme.win/trigger -H 'Content-Type: application/json' -d '{"triggerId": "101deadbeefcafe133769420", "text": "Hello, World!"}'

works just fine. Problem is when I try to code it with an ESP32, I consistently get a 400 Bad Request error back.

Using Wokwi.com as a simulator, this is the code I have:

#include <WiFi.h>
#include <HTTPClient.h>
#include <ArduinoJson.h>

const int motion_sensor = 23;

const char* ssid = "Wokwi-GUEST";
const char* password = "";
const char* trigger_id = "101deadbeefcafe133769420";
const char* greeting = "Hello, World!";

const char* pushme_url = "https://pushme.win/trigger/";

unsigned long lastTime = 0;
unsigned long timerDelay = 5000;

void setup() {
  pinMode(motion_sensor, INPUT);
  Serial.begin(115200); 

  WiFi.begin(ssid, password);
  Serial.println("Connecting");
  while(WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println("");
  Serial.print("Connected to WiFi network with IP Address: ");
  Serial.println(WiFi.localIP());
}

void loop() {
  const int motion = digitalRead(motion_sensor);
  Serial.println(motion);
  delay(100);
  if ((millis() - lastTime) > timerDelay) {
    if (WiFi.status() == WL_CONNECTED) {
      if (motion == HIGH) {
        WiFiClient client;
        HTTPClient http;

        http.begin(client, pushme_url);

        http.addHeader("Content-Type", "application/json");

        StaticJsonDocument<200> doc;
        doc["triggerId"] = trigger_id;
        doc["text"] = greeting;

        String request_body;
        serializeJson(doc, request_body);
        int http_response_code = http.POST(request_body);

        Serial.print("HTTP Response code: ");
        Serial.println(http_response_code);
        
        http.end();
      }
    } else {
      Serial.println("WiFi Disconnected");
    }
    lastTime = millis();
  }
}

How would I remedy this?