Mail Flow Rule to add X-Header to outgoing message

We use a mail encryption product (EncryptTitan) with some of our clients.

In order to trigger mail encryption, our users add the phrase "send-secure" to the end of their subject line.

In Microsoft Exchange/MS365, we have a mail-flow rule that detects that the message is being sent to someone "outside the organization" and contains that phrase "send-secure" in the subject line, and it adds a special X-HEADER (

X-ETVALTOK) with an authorization token to the message.   
When the message reaches our mail security edge server (SpamTitan), it sees that the X-ETVALTOK smtp header is set, and it redirects the email over to the EncryptTitan servers for handling.
EncryptTitan validates that the token provided is legit and matches it to a specific tenant.
This works great in Exchange and MS365.   
Now how do I accomplish this for my Groupwise clients?
Not seeing any options anywhere similar to this in Groupwise/ Groupwise Admin???