curl --request PUT \
  --url https://api.hubapi.com/crm/objects/2025-09/{objectType}/{objectId}/associations/{toObjectType}/{toObjectId} \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
[
  {
    "associationCategory": "HUBSPOT_DEFINED",
    "associationTypeId": 123
  }
]
'

{
  "createdResourceId": "<string>",
  "entity": {
    "fromObjectId": "<string>",
    "fromObjectTypeId": "<string>",
    "labels": [
      "<string>"
    ],
    "toObjectId": "<string>",
    "toObjectTypeId": "<string>"
  },
  "location": "<string>"
}