Add identifier to subject of all emails

Hi all,

I was wondering how I can accomplish the task of haveing a specific word in
the subject of all of my emails to identify the company that I am writing
emails from. For example, when I bring up a ne email, I want to be able to
have something like [test] come up before I type in the subject of my email.
Is this possible? I was able to set up a message template for this, however,
how can I make this work for replies? Also, if the only was I can do this is
to create a template, can I add a shortcut to the template as a button on my
toolbar? I usually have my folder list view so a button in my toolbar would
be helpful. Please let me know if there is a best way to do this.

Thanks!
Lauren