I just accidentally discovered you can now directly PM an alias from their board post.  Hover your mouse over their alias name, it will highlight with an underline, and then click on it to bring up a message window.

I used an alias here so you can try it out.

You must be in the new version of TER. The classic version does not support it. Additionally, you can just cut and paste the alias name or simply type it into the "TO:" box.