Rule to modify subject line?

[JTinVA]

I am using a rule that forwards any outgoing message to another e-mail account and it works fine. However, I'd like to add in a twist ... whenever the message is sent, I need to have the subject line modified to include "\complete" at the end. Is there any way to do this? I see something for custom actions but don't know how to set something like this up.

Thanks a lot.