How do I get a webhook when a package's tracking status changes?

curl -X PUT https://api.lastscanned.com/v1/webhook \
  -H "Authorization: Bearer lsk_live_YOUR_KEY" \
  -H "Content-Type: application/json" \
  -d '{"url": "https://example.com/lastscanned"}'
# the response has your signing secret (whsec_...)

curl -X POST https://api.lastscanned.com/v1/watches \
  -H "Authorization: Bearer lsk_live_YOUR_KEY" \
  -H "Content-Type: application/json" \
  -d '{"trackingNumbers": ["1ZG2242W6822298280", "541270563758"], "metadata": {"order": "1042"}}'

import hashlib, hmac, os, time
from flask import Flask, abort, request

app = Flask(__name__)

@app.post("/lastscanned")
def lastscanned():
    parts = dict(p.split("=", 1) for p in request.headers["Lastscanned-Signature"].split(","))
    body = request.get_data()
    expected = hmac.new(os.environ["LASTSCANNED_WHSEC"].encode(), parts["t"].encode() + b"." + body, hashlib.sha256).hexdigest()
    if abs(time.time() - int(parts["t"])) > 300 or not hmac.compare_digest(expected, parts["v1"]):
        abort(401)
    event = request.get_json()
    watch = event["data"]["watch"]
    if event["type"] == "tracking.updated":
        result = event["data"]["result"]
        print(watch["trackingNumber"], result["status"], result["statusDescription"], watch["metadata"])
    elif event["type"] == "watch.ended":
        print("done", watch["trackingNumber"])
    return "", 200  # answer 2xx within 10 seconds