Update Contact via API not updating custom field

[ian-deans]

Hi I am attempting to update a custom field via an API call. I receive the 204 status code but when I look at my contact on the webapp or by using the API, no data has been modified.

https://api.hubapi.com/contacts/v1/contact/vid/151/profile?hapikey=secretstuff
{"properties": [
    { "property": "idm_id", "value": "1234-abcd-567-efg"}
  ] }

I then receive a 204 No Content status code.
However, the field is still blank and shows no sign of any update made.

[#2]

NOTE: You must use an OAuth access token to create or to update an event. API keys are not supported

developers.hubspot.com

Create or update a timeline event

PUT /integrations/v1/:application‐id/timeline/event - Create or update a timeline event