curl -X POST 'https://api.resend.com/contact-properties' \
  -H "Authorization: Bearer $RESEND_API_KEY" \
  -H 'User-Agent: my-app/1.0' \
  -H 'Content-Type: application/json' \
  -d '{
  "key": "example-key",
  "type": "example-type",
  "fallback_value": "example-fallback-value"
}'

# POST /contact-properties (Resend API)
Base URL: https://api.resend.com
Auth: Authorization: Bearer <RESEND_API_KEY>
Last verified: 2026-10-20

> Create a new contact property

## Critical Gotchas
* All requests must include a `User-Agent` header (e.g. `User-Agent: my-app/1.0`) or the API returns HTTP 403.

## cURL
curl -X POST 'https://api.resend.com/contact-properties' \
  -H "Authorization: Bearer $RESEND_API_KEY" \
  -H 'User-Agent: my-app/1.0' \
  -H 'Content-Type: application/json' \
  -d '{"key":"example-key","type":"example-type","fallback_value":"example-fallback-value"}'

## Parameters
| Name | In | Type | Required | Description |
| :--- | :--- | :--- | :--- | :--- |
| key | body | string | Yes | The property key. Max length is 50 characters. Only alpha... |
| type | body | string | Yes | The property type. |
| fallback_value | body | string | No | The default value to use when the property is not set for... |

## Response (200 OK)
{"id":"78261eea-8f8b-4381-83c6-79fa7120f1cf","object":"contact_property"}

---
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POST

POST /contact-properties — Resend API

Resend API

Create a new contact property

Base URL: https://api.resend.com
Auth: Authorization: Bearer <RESEND_API_KEY>
Last verified: 2026-10-20 · upstream hash matched

Actions Try in browser→

Agents: curl -H "Accept: text/markdown" this URL
→ 203 tokens · Vary: Accept

Critical gotchas

All requests must include a User-Agent header (e.g. User-Agent: my-app/1.0) or the API returns HTTP 403.

cURL

curl -X POST 'https://api.resend.com/contact-properties' \  -H "Authorization: Bearer $RESEND_API_KEY" \  -H 'User-Agent: my-app/1.0' \  -H 'Content-Type: application/json' \  -d '{  "key": "example-key",  "type": "example-type",  "fallback_value": "example-fallback-value"}'

Parameters

Name	In	Type	Required	Description
key	body	string	Yes	The property key. Max length is 50 characters. Only alphanumeric characters and underscores are allowed.
type	body	string	Yes	The property type.
fallback_value	body	string	No	The default value to use when the property is not set for a contact. Must match the type specified in the type field.

Response 200 OK

{
  "id": "78261eea-8f8b-4381-83c6-79fa7120f1cf",
  "object": "contact_property"
}

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