curl --request POST \
  --url https://api.hubapi.com/crm/v3/objects/deals/splits/batch/upsert \
  --header 'Content-Type: application/json' \
  --header 'private-app: <api-key>' \
  --data '{
  "inputs": [
    {
      "splits": [
        {
          "percentage": 123,
          "ownerId": 123
        }
      ],
      "id": 123
    }
  ]
}'

{
  "completedAt": "2023-11-07T05:31:56Z",
  "requestedAt": "2023-11-07T05:31:56Z",
  "startedAt": "2023-11-07T05:31:56Z",
  "links": {},
  "results": [
    {
      "splits": [
        {
          "createdAt": "2023-11-07T05:31:56Z",
          "archived": true,
          "archivedAt": "2023-11-07T05:31:56Z",
          "propertiesWithHistory": {},
          "id": "<string>",
          "properties": {},
          "updatedAt": "2023-11-07T05:31:56Z"
        }
      ],
      "id": "<string>"
    }
  ],
  "status": "PENDING"
}