Create Property

Define a custom contact property. Once defined, you can store values for it on any contact via the properties field on Create Contact and reference it in personalization.

Endpoint

POST /properties

Returns: 201 Created

Headers

Header	Value	Required
Authorization	Bearer <your-api-key>	Yes
Content-Type	application/json	Yes

Body Parameters

Parameter	Type	Required	Description
field_key	string	Yes	Internal identifier used in contact.properties. Lowercase letters, digits, and underscores. Must start with a letter. Max 100 characters. Immutable after creation.
display_name	string	Yes	Human-readable label shown in the dashboard. Max 255 characters.
data_type	string	No	One of string (default), number, boolean, date. Immutable after creation.
description	string	No	Optional description. Max 500 characters.
fallback_value	string	No	Default used in merge tags when a contact has no value. Max 2000 characters.

Reserved field keys

id, email and unsubscribed shadow built-in contact columns and are rejected.

The remaining built-ins — the contact's name, phone number, external id and timestamps — are unreachable anyway: field_key must match ^[a-z][a-z0-9_]*$, so a camelCase key like phoneNumber fails validation before it can collide. Custom properties live in their own namespace under contact.properties, so a key such as customer_since never clashes with a built-in field.

Response

{
  "id": "01957e3a-4b5c-7d8e-9f0a-1b2c3d4e5f6a",
  "field_key": "lifetime_value",
  "display_name": "Lifetime Value",
  "data_type": "number",
  "description": "Aggregate revenue from this contact across all orders.",
  "fallback_value": "0",
  "created_at": "2026-03-08T12:00:00.000Z",
  "updated_at": "2026-03-08T12:00:00.000Z"
}

Field	Type	Description
id	string	Property ID
field_key	string	Internal identifier — use this as a key in contact.properties
display_name	string	Human-readable label
data_type	string	string, number, boolean, or date
description	string | null	Description
fallback_value	string | null	Default value for merge tags
created_at	string	ISO 8601 timestamp
updated_at	string	ISO 8601 timestamp

---

Examples

cURL

curl -X POST "https://api.arsel.sa/v1/properties" \
  -H "Authorization: Bearer be_your_api_key" \
  -H "Content-Type: application/json" \
  -d '{
    "field_key": "lifetime_value",
    "display_name": "Lifetime Value",
    "data_type": "number",
    "fallback_value": "0"
  }'

JavaScript

const response = await fetch("https://api.arsel.sa/v1/properties", {
  method: "POST",
  headers: {
    "Content-Type": "application/json",
    Authorization: "Bearer be_your_api_key",
  },
  body: JSON.stringify({
    field_key: "lifetime_value",
    display_name: "Lifetime Value",
    data_type: "number",
    fallback_value: "0",
  }),
});

const property = await response.json();
console.log(property.id);

Python

import requests

response = requests.post(
    "https://api.arsel.sa/v1/properties",
    headers={
        "Content-Type": "application/json",
        "Authorization": "Bearer be_your_api_key",
    },
    json={
        "field_key": "lifetime_value",
        "display_name": "Lifetime Value",
        "data_type": "number",
        "fallback_value": "0",
    },
)

print(response.json())

C#

using var client = new HttpClient();
client.DefaultRequestHeaders.Add("Authorization", "Bearer be_your_api_key");

var payload = new
{
    field_key = "lifetime_value",
    display_name = "Lifetime Value",
    data_type = "number",
    fallback_value = "0"
};

var json = System.Text.Json.JsonSerializer.Serialize(payload);
var content = new StringContent(json, System.Text.Encoding.UTF8, "application/json");
var response = await client.PostAsync("https://api.arsel.sa/v1/properties", content);
Console.WriteLine(await response.Content.ReadAsStringAsync());

PHP

<?php
$ch = curl_init("https://api.arsel.sa/v1/properties");
curl_setopt($ch, CURLOPT_HTTPHEADER, [
    "Content-Type: application/json",
    "Authorization: Bearer be_your_api_key"
]);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode([
    "field_key" => "lifetime_value",
    "display_name" => "Lifetime Value",
    "data_type" => "number",
    "fallback_value" => "0"
]));
$response = curl_exec($ch);
echo $response;
curl_close($ch);

---

Error Responses

409 Conflict

{
  "status_code": 409,
  "name": "conflict",
  "message": "Property with field_key \"lifetime_value\" already exists"
}

422 Reserved Key

{
  "status_code": 422,
  "name": "validation_error",
  "message": "Field key \"email\" is reserved for built-in contact columns."
}

422 Validation Error

{
  "status_code": 422,
  "name": "validation_error",
  "message": "field_key must start with a lowercase letter and contain only lowercase letters, digits, and underscores"
}