One way I have found to do this is with a little javascript.
In the URL try something like this (NOTE: remove the s p a c e s):
j a v a s c r i p t:void(window.open('http://www.externalwebsite.com'));
If you want to get fancy, you can control the way the window is opened:
j a v a s c r i p t:void(window.open('http://www.externalwebsite.com','','width=725, height=560, top=25, Left=25, menubar=0, status=0, location=1, titlebar=0, resizable=1, toolbar=0, scrollbars=1'));
Hope this helps.