You can't, really. Tab behavior is configured by the user. You can use
JavaScript to specify that a new window is opened, but not only is that no
guarantee that it will open in a tab instead of a new window, but security
will necessarily prompt the user as to whether the new window should be
opened. You can add a "target=_blank" attribute to a hyperlink, and that
will open a page in a new window, but not necessarily in a tab.