curl --request POST \
  --url https://app.nex.ai/api/developers/v1/records/{record_id}/relationships \
  --header 'Authorization: <api-key>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "definition_id": "789",
  "entity_1_id": "1001",
  "entity_2_id": "2002"
}
'

{
  "id": "5001",
  "definition_id": "789",
  "entity_1_id": "1001",
  "entity_2_id": "2002",
  "created_at": "2024-01-15T10:00:00Z"
}