According to the Kommo API documentation for Updating custom fields,
This method allows to update singular or multiple custom fields of an entity.
To update a singular custom field, you can add the field ID into the method URL (/api/v4/leads/custom_fields/{id}).
When updating multiple custom fields, an array of field objects is passed. In the case with a singular custom field, the field model is passed.
The URL that you used contains an ID, so it is updating a singular custom field. This means the array square brackets is not to be used.
Scrolling down to Request parameters section, here are the required field you need to send in the request:
If you do not send any of these properties, or send a property it wasn’t expecting to receive (like id), then validation will fail, thus you will get a 400 error.