curl --request PATCH \
  --url https://api.terminal49.com/v2/shipments/{shipment_id}/custom_fields/{api_slug} \
  --header 'Authorization: <api-key>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "data": {
    "type": "custom_field",
    "attributes": {
      "value": "<string>"
    }
  }
}
'

{
  "data": {
    "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
    "type": "custom_field",
    "attributes": {
      "api_slug": "<string>",
      "value": "<string>",
      "display_value": "<string>"
    },
    "relationships": {
      "entity": {
        "data": {
          "type": "shipment",
          "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a"
        }
      },
      "definition": {
        "data": {
          "type": "custom_field_definition",
          "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a"
        }
      }
    }
  },
  "links": {
    "self": "<string>"
  }
}