open: includes/functions.inc.php

find:

    // the code has been eneterd like this: [url]http://.../[/url]
    if(!isset ($attributes['default'])) return '<a href="'.htmlspecialchars($content).'">'.htmlspecialchars(shorten_url($content)).'</a>';
    // the code has been eneterd like this: [url=http://.../]Text[/url]
    return '<a href="'.htmlspecialchars ($attributes['default']).'">'.$content.'</a>';

replace with:

    // the code has been eneterd like this: [url]http://.../[/url]
    if(!isset ($attributes['default'])) return '<a href="'.htmlspecialchars($content).'" target="_blank">'.htmlspecialchars(shorten_url($content)).'</a>';
    // the code has been eneterd like this: [url=http://.../]Text[/url]
    return '<a href="'.htmlspecialchars ($attributes['default']).'" target="_blank">'.$content.'</a>';

It's only in bbcode "link"
As you wanted.