Sendy + Postmark Broadcast Streams (SMTP)

Hello,

Postmark have recently opened up the ability to use their services for 'broadcast' messages.

I can see that in Sendy I can setup SMTP as a method for sending msgs.

Here is what postmark say about setting up SMTP for them:

SMTP

To send with Broadcast Message Streams via SMTP, it requires the ability to add additional Headers with your SMTP client. The X-PM-Message-Stream header is your Broadcast Message Stream ID.

Server: smtp-broadcasts.postmarkapp.com
Ports: 25, 2525, or 587
Username: server token
Password: server token
Authentication: Plain text, CRAM-MD5, or TLS
Header: X-PM-Message-Stream: broadcasts

As you can see it requires the ability to add additional Headers with the SMTP client (sendy).

How can I configure this in Sendy to do this?

Thank you.