curl --request GET \
  --url https://api.example.com/api/v2/opportunity/{opportunity_id}/relationship/contacts \
  --header 'Authorization: Bearer <token>'

{
  "count": 123,
  "next_cursor": "<string>",
  "parent": {
    "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
    "record_type": "<string>"
  },
  "api_name": "<string>",
  "items": [
    {
      "related_record_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
      "related_record_type": "<string>",
      "through": {
        "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
        "record_type": "<string>",
        "attributes": {
          "is_primary": {
            "value": true,
            "value_type": "boolean"
          }
        }
      }
    }
  ]
}