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I hope you don’t mean to make new tabs the default behavior of the site. I prefer to be able to make the choice myself, via key commands, as already discussed.
But if you wish to include a link in your own post that will specifically open in a new window/tab, add the target attribute to the anchor tag:
<a href="[url]" target="new">Linked text</a>
Whether it opens a new window or a new tab may be determined by the user’s browser preference.