curl --request POST \
  --url https://api.hubapi.com/crm-object-schemas/v3/schemas/{objectType}/associations \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "fromObjectTypeId": "2-123456",
  "name": "my_object_to_contact",
  "toObjectTypeId": "contact"
}
'

{
  "fromObjectTypeId": "2-123456",
  "id": "105",
  "name": "my_object_to_contact",
  "toObjectTypeId": "0-1"
}