SMSTunnel for Uptime Kuma

Help me get Uptime Kuma alerts as text messages from my own phone, with SMSTunnel.

About SMSTunnel (https://smstunnel.io): my own apps and tools hand text messages (SMS) to my SMSTunnel account, and my own Android phone sends them from my own number, on my own SIM and plan. The phone runs the SMSTunnel app from Google Play (https://play.google.com/store/apps/details?id=io.smstunnel.app), signed in to my account.

What I want: [describe what should send a text, to whom, and when]

How it works:
- No plugin: Uptime Kuma's built-in Webhook notification calls SMSTunnel's API. Settings → Notifications → Set Up Notification, Notification Type "Webhook".
- Post URL: https://smstunnel.io/api/v1/sms/send (HTTP Method POST, if the field is shown).
- Request Body: "Custom Body": {"to": "+14155550123", "message": {{ msg | json }}} — {{ msg | json }} is Uptime Kuma's alert text, already JSON-quoted. The number goes in "to", in international format.
- Additional Headers (turn it on): {"Content-Type": "application/json", "X-API-Key": "<my SMSTunnel key>"} — the Content-Type line is needed so SMSTunnel reads the body as JSON.
- One notification per phone number: SMSTunnel sends one text to one number per request. Attach the notifications to monitors in each monitor's Notifications list, or with "Default enabled" and "Apply on all existing monitors".
- Without a key: an SMSTunnel webhook address, made in my dashboard with Uptime Kuma picked, which sets the webhook's text of the message to {{msg}} (without it, the monitor's name shows twice). In the Webhook notification: Post URL = the address, Request Body "Preset - application/json", no Additional Headers. One notification texts up to 10 numbers saved in the webhook.
- Without a key: an SMSTunnel webhook address, https://smstunnel.io/api/v1/webhook/<token>. I make it in my dashboard (https://dashboard.smstunnel.io): Apps → Webhooks → New webhook, pick the tool that will call it (Grafana, Uptime Kuma, Healthchecks, Gatus, Alertmanager, Odoo Online, or Something else), add 1 to 10 numbers (each gets its own text), optionally a text of the message that pulls fields out of the JSON with {{path}}, like {{alarm.name}}, then Create the webhook and copy the address. A GET with ?message=… or a POST with JSON (its message, text, title or body field) texts those numbers; the call never sets the number. Texts are cut at 480 characters. "Calls an hour" (20 unless I set another number, up to 500) counts calls, not texts: each call texts every number, and calls past it are refused until the hour is over. The address can be copied again under Address and setup; New address replaces it if it leaks. Scripts can make one with POST https://smstunnel.io/api/v1/webhooks and my sign-in token instead.
- The Test button sends "<Friendly Name> Testing". A wrong key shows 401 "Invalid or expired API key"; 429 means too many requests (SMSTunnel allows 30 sends a minute).
- Alerts look like "[My shop] [🔴 Down] timeout of 48000ms exceeded" and "[My shop] [✅ Up] 200 - OK". A text can be up to 1600 characters. If my phone is offline, the text waits and goes out when it is back.
- The API key comes from my SMSTunnel dashboard (https://dashboard.smstunnel.io): Apps → Add an app → Create the key. It is shown only once and can send texts and check whether they went out.

Rules for your answer:
- Do not ask for my SMSTunnel password or API key and do not put a real key in your answer; I will not paste it in this chat.
- Phone numbers must be in international format (+ and the country code).
- Help me test it with my own number first.
- Only send texts people expect from me. No unsolicited texts.
- Help me avoid floods of texts from a monitor that keeps going up and down (Retries, resend settings).