curl --request POST \
  --url https://api.hubapi.com/crm/objects/v3/projects/merge \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "objectIdToMerge": "<string>",
  "primaryObjectId": "<string>"
}'

{
  "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"
}