curl -X GET https://api.mailofly.com/contact-properties \
  -H "Authorization: Bearer mf_live_your_api_key"

{
  "data": [
    {
      "id": "prop_123",
      "name": "plan_tier",
      "type": "string",
      "label": "Plan Tier"
    },
    {
      "id": "prop_456",
      "name": "signup_date",
      "type": "date",
      "label": "Signup Date"
    }
  ]
}