Schedule Single Send

**This endpoint allows you to send a Single Send immediately or schedule it to be sent at a later time.** To send your message immediately, set the `send_at` property value to the string `now`. To schedule the Single Send for future delivery, set the `send_at` value to your desired send time in [ISO 8601 date time format](https://www.iso.org/iso-8601-date-and-time-format.html) (`yyyy-MM-ddTHH:mm:ssZ`).