It looks like the forum software doesn’t support this - and it’s intentional.
I’m not sure that I fully agree, but the argument goes that many people find enforced new browser windows rather annoying. Some don’t notice that a new window has opened, and if they are used to clicking on the back button, they may be confused by the fact that they can’t back out (the back button is grayed out). Some also may not understand that they must close the new window to return to the original content. In addition, if a pop-up blocker on the visitor’s PC prevents a new window from opening, the visitor may assume the link is broken. By default, some browsers prevent new windows from being opened.
Here is a more extreme view that I spotted on the web recently:
Opening up new browser windows is like a vacuum cleaner sales person who starts a visit by emptying an ash tray on the customer’s carpet - if I want a new window, I will open it myself!
Designers open new browser windows on the theory that it keeps users on their site. But even disregarding the user-hostile message implied in taking over the user’s machine, the strategy is self-defeating since it disables the Back button which is the normal way users return to previous sites. Users often don’t notice that a new window has opened, especially if they are using a small monitor where the windows are maximized to fill up the screen. So a user who tries to return to the origin will be confused by a grayed out Back button.
Because of all this, the HTML code that is commonly used to force a link to open in a new browser window is no longer considered valid in the HTML 4.0 Strict and XHTML 1.0 Strict recommendations of the W3C.
Anyway, it’s noted as a requested feature for possible future inclusion.