curl -X POST https://console.trychert.com/api/v1/chats/ch_01H.../participants \
  -H "x-chert-tenant: $TENANT" \
  -H "x-chert-signature: v1,$TS,$SIG" \
  -H "content-type: application/json" \
  -d '{"address": "+14155551234"}'

const ts = Math.floor(Date.now() / 1000)
const body = JSON.stringify({ address: "+14155551234" })
const sig = crypto.createHmac("sha256", SIGNING_SECRET).update(`${ts}.${body}`).digest("hex")
const res = await fetch(
  `https://console.trychert.com/api/v1/chats/${chatId}/participants`,
  {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
      "x-chert-tenant": TENANT_SLUG,
      "x-chert-signature": `v1,${ts},${sig}`,
    },
    body,
  },
)

import hmac, hashlib, json, time, requests
ts = int(time.time())
body = json.dumps({"address": "+14155551234"})
sig = hmac.new(SIGNING_SECRET.encode(), f"{ts}.{body}".encode(), hashlib.sha256).hexdigest()
r = requests.post(
    f"https://console.trychert.com/api/v1/chats/{chat_id}/participants",
    headers={
        "Content-Type": "application/json",
        "x-chert-tenant": TENANT_SLUG,
        "x-chert-signature": f"v1,{ts},{sig}",
    },
    data=body,
)

{
  "chat_id": "ch_01H...",
  "added": "+14155551234",
  "participants": ["+14155550100", "+14155550200", "+14155551234"]
}