From the MVP site and David Bartosik's FAQ page at:

http://www.publishermvps.com/Default.aspx?tabid=30

Q.  How do I make my hyperlink open the URL in a new browser window?
A.  Launching a new window is done by using the target attribute of the
link. Publisher does not have any support for this. Meaning there is no way
to code a target when linking within Publisher. The workaround is to use the
HTML Code Fragment feature and code the link yourself, including the desired
target. As such:
To open a new window per target:

<a target="_blank" href="http://www.domain.com">Link Text</a>

To re-use the window you launched on each target use:

<a target="_display" href="http://www.domain.com">Link Text</a>

DavidF