Add message-subject to AS2 encoded message using Azure logic app

I've created an Azure logic app that encodes some xml in an AS2 message, then sends this message to another server.

Flow:

1. Trigger[HTTP] ->
2. AS2 Encode body of trigger ->
3. Add AS2 headers and encoded body to an HTTP POST message.

It works with the default options but now I have been asked to add the AS2 header "message-subject", which is not currently available via the Logic App builder.

How do I add the AS2 header "message-subject" to my HTTP message, using Azure logic apps?