curl --request POST \
  --url https://app.fliqr.ai/api/users/{user_id}/custom_fields/{custom_field_id} \
  --header 'Content-Type: application/x-www-form-urlencoded' \
  --header 'X-ACCESS-TOKEN: <api-key>' \
  --data 'value=<string>'

{
  "success": true
}