Add custom domain

A valid request URL is required to generate request examples

{
  "domain": {
    "id": 123,
    "domain": "sign.example.com",
    "primary": true,
    "verification_record_name": "<string>",
    "verification_record_value": "<string>",
    "ssl_status": "<string>",
    "dns_verified_at": "2023-11-07T05:31:56Z",
    "ssl_provisioned_at": "2023-11-07T05:31:56Z",
    "last_checked_at": "2023-11-07T05:31:56Z",
    "last_error": "<string>",
    "created_at": "2023-11-07T05:31:56Z"
  }
}