curl --request POST \
  --url https://api.interchange.io/api/v2/buyer/advertisers/{advertiserId}/test-cohorts \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "name": "West Coast Treatment Group",
  "cohortType": "geographic",
  "definition": {
    "type": "zip_code"
  }
}
'

{
  "cohort": {
    "id": "cohort_abc123",
    "advertiserId": "12345",
    "name": "West Coast Treatment Group",
    "cohortType": "geographic",
    "definition": {
      "type": "zip_code"
    },
    "isActive": true,
    "createdAt": "2025-01-15T10:30:00Z",
    "updatedAt": "2025-01-20T14:45:00Z",
    "description": "<string>",
    "estimatedSize": 50000
  }
}