curl -X POST https://api.mailofly.com/api/v1/contact-properties \
  -H "Authorization: Bearer mf_live_your_api_key" \
  -H "Content-Type: application/json" \
  -d '{
    "name": "plan_tier",
    "type": "string",
    "label": "Plan Tier"
  }'

{
  "data": {
    "id": "prop_123",
    "name": "plan_tier",
    "type": "string",
    "label": "Plan Tier",
    "createdAt": "2026-09-11T13:40:00Z"
  }
}