# Upsert a custom field value on a track

Creates or updates the value for the given custom field key. A blank/null fieldValue clears the existing value. Returns 204 on success.

Endpoint: POST /integration/rest/api/v1/track/{trackId}/custom-field
Version: v0
Security: X-AUTH-KEY

## Path parameters:

  - `trackId` (string, required)

## Request fields (application/json):

  - `fieldKey` (string, required)
    System key (name) of the custom field; must match a configured field.
    Example: "CARRIER"

  - `fieldValue` (string)
    Value to set. For NUMERIC fields send the number as a string; for DATE fields send an ISO-8601 date-time. A blank/null value clears the existing value.
    Example: "DHL Express"

## Response 400 fields (*/*):

  - `code` (string)
    Error code

  - `message` (string)
    Error message

  - `timeStamp` (integer)
    Timestamp

  - `correlationId` (string)
    Correlation id

## Response 404 fields (*/*):

  - `code` (string)
    Error code

  - `message` (string)
    Error message

  - `timeStamp` (integer)
    Timestamp

  - `correlationId` (string)
    Correlation id

## Response 500 fields (*/*):

  - `code` (string)
    Error code

  - `message` (string)
    Error message

  - `timeStamp` (integer)
    Timestamp

  - `correlationId` (string)
    Correlation id