curl --request PUT \
  --url https://api.hubapi.com/crm/v4/objects/{objectType}/{objectId}/associations/{toObjectType}/{toObjectId} \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
[
  {
    "associationCategory": "HUBSPOT_DEFINED",
    "associationTypeId": 123
  }
]
'

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