curl --request GET \
  --url https://api.embedreach.com/api/communication-groups/{id}/merge-fields

{
  "success": true,
  "message": "<string>",
  "data": {
    "mergeFields": [
      {
        "category": "<string>",
        "entries": [
          {
            "label": "<string>",
            "value": "<string>"
          }
        ]
      }
    ]
  }
}