curl --request POST \
  --url https://api.hubapi.com/crm/v3/objects/users \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "associations": [
    {
      "types": [
        {
          "associationCategory": "HUBSPOT_DEFINED",
          "associationTypeId": 123
        }
      ],
      "to": {
        "id": "<string>"
      }
    }
  ],
  "properties": {}
}'

{
  "location": "<string>",
  "createdResourceId": "<string>",
  "entity": {
    "archived": false,
    "createdAt": "2019-10-30T03:30:17.883Z",
    "id": "512",
    "properties": {
      "property_checkbox": "false",
      "property_date": "1572480000000",
      "property_dropdown": "choice_b",
      "property_multiple_checkboxes": "chocolate;strawberry",
      "property_number": "17",
      "property_radio": "option_1",
      "property_string": "value"
    },
    "updatedAt": "2019-12-07T16:50:06.678Z"
  }
}