curl --request POST \
  --url https://api.example.com/public/contacts/{contact_id}/custom-fields \
  --header 'Content-Type: application/json' \
  --data '
{
  "field_name": "<string>",
  "field_value": "<string>"
}
'

{
  "id": 123,
  "updated_time": "2023-11-07T05:31:56Z",
  "field_name": "<string>",
  "field_value": "<string>"
}